Distributed Feedback Random Fiber Laser Based On Raman Gain

In recent years,random lasers represent a rapidly growing class of light sources,in which the conventional optical cavity have been replaced by multiple scattering feedback in disordered gain media(such as laser crystals or semiconductor powders).This random laser has good stability?small size?low price and other advantages.Although random lasers have many interesting characteristics,most of them lack the basic characteristics of lasers,such as directivity and high power.In order to overcome its shortcomings,researchers have proposed a variety of random lasers based on low-dimensional structures,such as multilayer structures?hollow-core photonic crystal fiber structures and waveguide arrays.And random lasers based on fiber are considered to be superior to other types of random lasers,because the fiber waveguide structure is almost one-dimensional,which can well limit the radial scattering of light and control the output light on the fiber axis.Random fiber laser has the characteristics of good directivity and high output power,which fundamentally solves the problem of traditional random lasers.The output characteristics of distributed feedback random fiber laser based on Raman gain have more output characteristic than the conventional Raman fiber laser in many aspects,especially the higher first-order efficiency.In terms of cascaded generation,it has the characteristics of larger tuning range,higher spectral flatness of multi-wavelength generation,narrower bandwidth and higher conversion efficiency of nonlinear crystal.The unique performance of distributed feedback random fiber laser based on Raman gain provides a new platform for laser physics?nonlinear optics?disordered system theory and other scientific research areas and opens the door for their application in advanced technology,such as distances without amplifier transmission and sensing,visible light source for low-coherence infrared and speckle-free biological imaging,laser display,etc.In this paper,the experimental study of distributed feedback random fiber lasers based on Raman gain is carried out.The contradiction between the laser threshold and the cavity length is resolved by using highly nonlinear fibers(HNLF),and a series of researches on the cavity design and output characteristics of distributed feedback random fiber laser based on Raman gain were studied.In addition,the evolution of the h-state pulse with power and polarization state was demonstrated for the first time through experiments in the Raman fiber laser because of the ultra-high nonlinearity and Raman gain of HNLF.Finally,a sub-microsecond Q-switched light source based on black phosphorus at the communication band was designed in order to explore the difference between the mechanism and output characteristics of random laser generated by pulse pumping and continuous optical pumping.The several research results of this paper are as follows:1?A simple and compact single-mode high-power Er/Yb co-doped all-fiber laser is designed as the pump source of distributed feedback random fiber laser based on Raman gain.The contradiction between the laser threshold and the cavity length is solved by using high nonlinear fiber(HNLF).The laser threshold of half-opened distributed feedback random fiber laser based on Raman gain is as low as 0.408W,which is the minimum threshold reported so far.The working wavelength of the laser is 1658 nm,which is the first operating of distributed feedback random fiber laser based on Raman at this wavelength.The output characteristics under different lengths HNLF were studied.The laser threshold and optical conversion efficiency show a decreasing trend with the increase of the fiber length when the HNLF does not exceed850 m.However,when the HNLF is 950 m,the output characteristics will be completely different.The spectral evolution process is similar to that of the open distributed feedback random fiber laser based on Raman gain,with a laser threshold of 1.111 W and a slope efficiency of 32%.It provides a new idea and basis for the realization of short cavity and low threshold distributed feedback random fiber laser based on Raman gain.2?The ring cavity distributed feedback random fiber laser based on Raman gain was studied.When HNLF does not exceed 850 m,only the resonance effect of the ring cavity forms the feedback,that is,the output laser is the traditional continuous Raman laser.When HNLF is 950 m,the laser feedback is completely provided by random distributed Rayleigh scattering.The laser threshold value of the ring cavity distributed feedback random fiber laser based on Raman gain is 0.284 W,which is about 1/4 of the full cavity(1.111 W).compared with the linear cavity,distributed feedback caused by Rayleigh scattering and the cavity effect that leads to random distributed feedback will coexist in the ring cavity fiber laser,which will greatly reduce the laser threshold.However,the optical power distribution of the two cavity structures is different,resulting in a certain difference in spontaneous radiation probability.The optical efficiency(15%)of the ring cavity distributed feedback random fiber laser based on Raman gain is lower than that of the full open cavity(32%).The experimental results show that the random fiber laser based on Raman gain can provide a new idea for the random laser with low threshold.3?The gain of distributed feedback laser random fiber laser and Raman fiber laser are both obtained by stimulated Raman scattering in the fiber.It is found that the output of the ring cavity random fiber laser has modeless.In order to understand the effect of stimulated Raman effect on pulsed laser,we research on pulsed Raman fiber laser.We experimentally demonstrated the generation of h-shape pulses for the first time in the Raman fiber laser mode-locked by using NPR technology.The total cavity length was 632 m and the laser were operated at negative dispersion with-2.47 ps~2.The h-shape pulse shows both pump power and polarization state(PS)related evolution.The h-shape pulse can broaden up to 270 ns without spilt.The peak power of h-shape pulse decreases gradually and tends to a fixed value,which reveals that the peak power clamp effect is enhanced continuously.Besides,the h-shape pulse also can operate at harmonics by varying PS.The experiments deepened our understanding of the h-shape pulse and provide a method to achieve high-energy pulses in the Raman fiber laser.4?In order to explore the difference between the mechanism and output characteristics of random laser generated by pulse pumping and continuous optical pumping.But the laboratory lacks a narrow pulse source in the communication band,so a sub-microsecond Q-switched fiber laser light source based on black phosphorus was studied.A compact bait-doped all-fiber ring-cavity laser achieves sub-microsecond Q-switched pulses.The layered black phosphorus material was prepared by mechanical peeling method,and the non-linear optical characteristics of the layered black phosphorus after peeling were measured by balanced synchronous dual detection method.Based on this characteristic,a saturable absorber based on black phosphorus was prepared.In the fiber laser,a stable Q-switched pulse output with a center wavelength of 1557.9 nm and a shortest pulse duration of 742 ns is realized.The experimental results show that the layered black phosphorus can be used as an effective saturable absorber for generating sub-microsecond Q-switched pulses.Moreover,after amplification,this Q-switched light source can be used to pump HNLF to optimize Raman threshold,and explore the use of pulse-pumped at the communications band to generate the long wavelength pulse random laser.