Research On Key Technologies Of Miniaturized Femtosecond Fiber Optical Frequency Comb And Wavelength Conversion

Owing to their advantages of wide optical spectrum range and high frequency stability,femtosecond optical frequency combs are widely used in important research fields such as optical frequency measurement,absolute distance measurement,precise spectrum measurement,atomic clock network,and low noise microwave signal generation.Compared with other femtosecond optical frequency combs,erbium-doped fiber optical frequency combs based on all polarization-maintaining fiber and components have many advantages such as compact structure,easy maintenance,compatibility of working wavelength with optical communication band,and strong environmental adaptability,which have become a research and application hotspot in recent years.In this paper,we experimentally studied the key technologies of the generation and amplification of femtosecond laser pulses based on all polarizationmaintaining erbium-doped fiber,the frequency detection and locking of the comb's frequencies,and the nonlinear wavelength conversion of femtosecond optical comb output laser,and explore the application of compact,high-performance,robust,application-oriented femtosecond fiber optical frequency comb.The main work and innovations of this paper are summarized as follows:1.A 200-MHz all polarization-maintaining femtosecond erbium-doped fiber optical frequency comb system is designed and constructed.In femtosecond oscillator design,we employ a linear laser cavity structure,and self-mode-locking is achieved by using an SESAM.The laser oscillator delivers mode-locked pulse train with the central wavelength of 1560.7 nm,and an average output power 3.9 m W.By injecting the compressed 53-fs,12.5-k W femtosecond pulse into highly nonlinear optical fiber,an octave broadband supercontinuum with a spectral range of 1000 nm~2050 nm is generated.Furthermore,through the self-reference collinear "f-2f" interference technique,an fceo signal with a signal to noise ratio of 40 d B is obtained.Finally,the repetition frequency and carrier-envelope offset frequency are locked simultaneously through feedback adjustment of the oscillator cavity length and pump current.When referenced to a hydrogen maser,the relative frequency stability of repetition rate in 7200 s is 1.65?10-12 at a gate time of 1 s,and the residual integration timing jitter reaches 418 fs [3 Hz to 1 MHz].Within 5-h frequency counting time,the relative frequency stability of the carrier-envelope offset frequency is 3.22?10-11/s,and the integrated phase noise is 0.216 rad in the integrated bandwidth of 100 Hz to 1 MHz.2.A two-color visible comb light source based on 1.5-mm all polarization-maintaining fiber laser is designed and constructed.By optimizing the parameters of the amplifier and the highly nonlinear optical fiber(zero-dispersion wavelength,dispersion slope and length),the maximum spectral energy distribution are achieved at 1086 nm and 1266 nm.After that,4-cm long Mg O doped periodically poled lithium niobate crystals with different polarization periods are used to generate 543 nm and 633 nm frequency doubled lasers at 112 ? and 59 ?,respectively.The spectral bandwidths of the 543-nm and 633-nm comb lights are 0.157 nm and 0.174 nm.The average powers of single comb tooth are 1.23 mW and 1.336 mW,respectively.This two-color visible comb with narrow spectrum bandwidth and high average power of single comb tooth is beneficial for the improvement of signal-to-noise of beat signal in optical frequency measurement.3.A novel fiber frequency actuator with high control bandwidth and large adjustment range for repetition rate locking is designed and developed.Compared with the traditional cavity length adjusting unit,the frequency actuator is a fiber stretching unit based on single piezoelectric ceramic and a precise mechanical unit,which has no damage to the intracavity fiber.In the case of using the frequency actuator to modulate the cavity length of the oscillator,a systematic experimental study on the dynamic and static response characteristics of the repetition frequency is carried out.The static response curve shows that the tuning range of the frequency actuator is up to 106 k Hz.The dynamic response results show that the control bandwidth of the frequency actuator is about 1 k Hz.The frequency actuator with high control bandwidth and large tuning range can significantly reduce the power consumption and volume of the optical frequency comb.4.The engineering prototype of the optical frequency comb with compact structure,high integration and high environmental adaptability has been explored and developed.A fiber-coupled,gradient index lens-based micro "f-2f" self-referenced interferometer with a diameter of 4 mm and a length of 20 mm is designed and realized.Compared with the traditional multi-lens spatial interferometer structure,the interferometer has the advantages of small size and low cost.Focus on the application of optical frequency comb in the field of outdoor high-precision distance measurement,we have successfully developed a 200-MHz all polarization-maintaining,repetition-rate-locked femtosecond fiber laser by using a laboratorymade fiber frequency actuator and digital programmable gate array frequency stabilization technology.The power consumption of the laser is only 11 W,the volume is less than 0.0015 m3,and the weight is only 1.3 kg.In the vibration and shock environment with a peak acceleration of 1.97 g,the stability of the repetition frequency is not affected after locking.In addition,the frequency stability of the laser system is better than 2.0?10-11/s at 15 ? and 33 ?.This highly integrated,robust repetition-rate-locked frequency comb is of great significance for highprecision absolute distance measurement in outdoor environment,even outer space environment.