The Influence Of The Distribution Of Refractive Index And Amplification On Lasing Modes Generating From A Random Amplification System

Random laser is a kind of laser with a special working principle and unique radiation characteristics.Regardless of the absence of a resonant cavity,a random laser can produce good modes of laser radiation.It also owns both spatial divergence and highly time coherence which are mutually exclusive with each other in a traditional cavity laser.These unique characteristics lead to a great potential in the development of micro-nano photonics.However,the mechanism for generating laser modes and effective control over the lasing modes are still a basic question to be answered in the research area of random laser.In this article,we studied the optical feedback provided by the distribution of amplification besides the one provided by random scattering,which gives us a further understanding of the mechanism of mode generation in random laser and the possibility to obtain control over random laser modes.Random laser can be achieved by pumping dye-doped liquid crystal film.More interestingly,the random laser excited in dye-doped liquid crystal film have a strange lasing spectrum,in which the modes are uniformly-spaced and the mode spacing is determined by the thickness of the sample.Though this phenomenon reminds us a resonant cavity laser,the reflection at the border of samples couldn't provide enough feedback to form such laser mode.We think that the distribution of amplification within the sample might provide some kind of feedback when pumping,making the mode characteristics of laser radiation change from random to cavity-like.Without a reflection feedback,a laser radiation with uniformly-spaced mode distribution was excited by designing an amplification structure,which could form a feedback similar to the reflection feedback.This optical feedback provided by amplification distribution could be enhanced by decreasing the scattering intensity or by controlling the shape of the pump light to form an ideal linear pumping area.The experiment results show that both amplificationdistribution-induced feedback and random-scattering-induced feedback have an influence on the lasing modes during the exciting process of lasing in dye-doped liquid crystal film,and there is competition in the intensity of the two feedback mechanism,which leads to the changing of the lasing mode characteristics.A related calculating result using FDTD algorithm are also present.By combining the Maxwell equations and the rate equation of a four-level system,we construct a mathematical model to simulate the steady-state pump-induced light emission from a scattering amplification structure.The calculating results show that the influence of optical feedback provided by the distribution of amplification is similar to a resonant cavity,and the mode characteristics of the laser from the random gaining media can be changed by turning the balance between the feedback induced by amplification distribution and the one induced by random scattering or by using high pump energy which may break the amplification structure.In the end,an amplification-modulated structure was developed in calculation,which may be used to control laser modes in a mechanical way.