Linearly polarised three-colour lasing emission from an evanescent wave pumped and gain coupled fibre laser

A novel Whispering--Gallery--Mode (WGM) fibre laser,emitting linearly polarised three-colour light, is demonstrated bypumping and gain coupling with evanescent waves. The pump light islongitudinally coupled into a bare optical fibre immersed in a dyesolution of lower refractive index. The dye molecules around thebare fibre are excited by the evanescent waves of pump light whenthey propagate along the fibre in a total internal reflection. Whenthe pump beam within the fibre is a meridian beam, the WGM lasingemission from the fibre laser is a linearly polarised transverseelectric wave, while it is a mixed wave of the linearly polarisedtransverse electric and magnetic waves if the pump beam is a skewbeam within the fibre. Because the excited molecules are locatedwithin the evanescent field of WGM, a good spatial overlap betweenthe dye gain and the evanescent field leads to a high pumpingefficiency and a longer gain distance along the fibre. Once the barefibre is inserted into three glass capillaries filled with Rhodamine6G, 610 and 640 dye solutions, respectively, WGM laser oscillationsat the wavelengths of 567--575, 605--614 and 656--666~nm occursimultaneously, and a linearly polarised three-colour lasingemission is achieved in a single optical fibre.