Synchronously pumped ring fiber raman laser

Abstract

An all-fiber ring laser has a single, uninterrupted length of single-mode optical fiber that is formed into a loop by using an optical coupler. Pump signal pulses at a first optical wavelength are introduced into one end of the optical fiber. Each pump pulse propagates through the loop formed in the fiber and then exits the fiber. The pump signal pulses excite the molecules of the optical fiber to cause them to go to a higher, unstable energy level. When the molecules return to a lower energy level, photons are emitted at a second optical frequency that has a wavelength that is shifted from the wavelength of the pump signal to form laser signal pulses. The coupler is a multiplexing coupler that has a first coupling ratio at the wavelength of the pump signal and has a second coupling ratio at the wavelength of the emitted optical signal. The first coupling ratio is preferably close to zero and the second coupling coefficient is greater than 0.5. The laser optical signal recirculates in the loop to stimulate further emission of optical energy at the shifted wavelength in phase with the recirculating optical signal. Each pump pulse is timed to enter the optical fiber loop in synchronism with the recirculating laser signal pulse so as to excite the fiber molecules so as to amplify the recirculating signal pulses. The laser signal has a loop transit time related to the time interval between pump pulses such that a multiple of the loop transit time is substantially equal to a multiple of the time interval.