Research On Key Technology Of All Fiber Structure Single Cavity Double Comb Laser Based On Multimode Interference

In 1999,the optical frequency comb was born based on the technology of ultrafast laser and femtosecond laser.One of its main applications is the dual-comb spectroscopy（DCS）technology,which uses two optical combs for asynchronous optical sampling.However,the system required for this technology is very complex,bulky,and expensive.Therefore,researchers have proposed a single cavity dual optical comb scheme for outputting asynchronous ultrashort pulses in the same laser cavity,and many experiments have achieved preliminary results.However,because single-cavity dual combs only use one laser resonator,redundant devices or pump sources are ultimately needed to convert a single pulse into a dual pulse,which means more inter-device interference and a lack of structural compactness.To further simplify the structure of a single cavity dual optical comb,this article aims to explore the possibility of combining mode-locking devices with wavelength multiplexing devices and explore an all-fiber structure single cavity dual optical comb laser scheme that still has high stability and asynchronous pulse output,while reducing the number of intracavity devices and improving the compactness.The following aspects of the research have been mainly carried out:1.This article analyzes and introduces several methods currently used to achieve single cavity and dual combs,most of which use two-dimensional materials for mode-locking assisted by wavelength/polarization multiplexing for output or achieve dual pulse output with dual pump and dual direction.Most of these schemes have low damage thresholds,requiring additional filtering/polarization devices,and some even require multiple sets of mode-locking devices.Based on the above analysis,this article explores the mode-locking principle based on the nonlinear multimode interference effect（NL-MMI）.It illustrates its advantages over traditional mode-locking methods:low mode-locking threshold,stronger damage resistance,and,more importantly,it also has filtering effects,laying a theoretical foundation for the combination of mode-locking devices and wave-filtering devices.2.To enable a single nonlinear multimode interference device to have both mode locking and filtering functions,this paper successfully proposes a new device:single mode fiber-tapered micro-nano fiber with graded refractive index-single mode fiber（T-SMS）by introducing a tapered micro-nano fiber that also has nonlinear multimode interference effects.Simulation software has successfully verified that it has stronger nonlinear multimode interference effects and filtering effects.After that,this article used this device to build a single cavity dual-comb laser,successfully achieving stable dual-wavelength output at 1533 nm and 1556 nm,with a 3 d B bandwidth of 2.16 nm and 8.14 nm,center frequencies of 39.8757 MHz and 39.8786 MHz,and a signal to noise ratio of about 60 d B.Frequency difference stability and coherence tests were conducted.3.To solve the problems of excessive continuous optical components and high mode-locking threshold that affect the output quality of single cavity and double optical combs in practical experiments,a hybrid mode-locking method that combines heterojunction materials with T-SMS is adopted in this paper.Bi₂Te₃and Sb₂Te₃are prepared into transverse heterojunctions and deposited dropwise on the surface of T-SMS.A hybrid mode-locking saturable absorber is successfully prepared and applied to the single cavity and double optical combs;The mode-locking threshold is significantly reduced,enabling dual-wavelength mode-locking operations at around 160 m W.At the same time,using a spectrometer to observe the output of a single cavity dual comb,the spectrum is significantly smoother,effectively reducing the continuous optical component.Instability measurements confirmed that the hybrid mode-locked laser maintains high-frequency difference stability and high coherence.