GHz Optical Frequency Comb Generation from a Filtering Cavity Based on a Free-running Fiber Laser
High repetition rate optical frequency combs,characterized by short pulse intervals(<1 ns)and large longitudinal mode spacing(>1 GHz),are crucial for precision measurements in fields such as gas detection,astronomical spectrograph calibration,and high-capacity optical communication networks.However,due to the limitations of cavity length,it is challenging to achieve GHz repetition rates with fiber-based optical frequency combs.This paper is dedicated to enhancing the repetition rate of all-fiber optical frequency combs in the 1.5 μm wavelength band.Firstly,the parameters of the Fabry-Pérot cavity and their impact on filtering performance,as well as their applicability to filtering free-running comb sources,were analyzed in detail from a theoretical perspective.Based on the principle of mode filtering of optical frequency combs,a mathematical model was established in MATLAB to numerically simulate the effects of cavity parameters such as cavity length,mirror reflectivity,and mirror curvature radius on cavity linewidth and side mode suppression ratio.The theoretical simulation results indicate that an optimized filter cavity design requires the cavity linewidth to be greater than the comb linewidth and to have an appropriate tolerance range for the locking system.Therefore,a compromise design is chosen between the locking tolerance range and side mode suppression ratio,appropriately reducing the side mode suppression ratio to cope with the frequency drift of free-running lasers,which is the starting point for the experimental design of mode filtering.Subsequently,the optical frequency comb mode filtering scheme was numerically simulated,avoiding the adverse effects of cavity design that could lead to high-order modes coinciding with optical frequency comb modes,thereby significantly reducing the side mode suppression ratio after filtering.Based on theoretical simulations,this paper designed and constructed a mode filter cavity with a cavity linewidth of 14 MHz(70 times the comb linewidth)and a cavity mirror reflectivity of 97%.Then,based on the electro-optic modulation Pound-Drever-Hall technique,an experiment was conducted to filter the modes of a fully fiber-locked laser oscillator with a repetition rate of 97.4 MHz.This achieved a significant increase in the optical frequency comb repetition rate from 97.4 MHz to 1.75 GHz and 1.65 GHz,with a minimum side mode suppression of 23 dB.The frequency uncertainty of the stabilized comb was 0.99 MHz,successfully addressing the frequency drift of free-running fiber combs.Based on the experiments,the paper further theoretically proved that the designed mode filter cavity can filter most single-cavity dual-comb systems.Finally,this paper conducted four sets of comparative experiments to study the effects of cavity higher-order modes and the original repetition frequency of the optical comb on filtering performance.The experiments showed that when higher-order cavity modes overlap with the desired suppressed comb modes,the suppressed modes will leak out and reduce the local side mode suppression ratio.Additionally,the higher the original repetition frequency of the optical comb,the higher the side mode suppression ratio that can be achieved.In addition to the above analysis,a method was also discovered to further increase the repetition rate based on the cavity free spectral range.For example,by selecting a cavity length that matches every other cavity fundamental mode with a comb tooth,the repetition rate of the filtered comb is twice the cavity free spectral range.This method is an effective way to obtain a high repetition rate optical frequency comb through a simpler mode filter cavity structure.The experiment in this paper successfully achieved a GHz repetition rate locked femtosecond laser.The above exploration may further realize a higher signal-to-noise ratio and more portable GHz fiber optical frequency combs.
Optical frequency combMode-filtering cavityRepetition frequency multiplicationFiber laserDual-comb spectroscopy
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