Scientists at the University of Central Florida in Orlando have developed a "breathing mode" semiconductor laser with mode-locked, externally compressed 274-fs pulses. The approach promises semiconductor lasers capable of producing even shorter pulses. Such lasers could be commercially important because of their small size, low cost and electrical efficiency.The breathing mode designation refers to the fact that, as the intracavity pulses circulate around the ring resonator, they are stretched before entering the gain medium and compressed before encountering the saturable absorber (see figure). The lower peak power of the stretched pulses minimizes the amount of self-phase modulation generated in the gain medium.The intracavity pulses of the "breathing mode" ring laser are alternately stretched and compressed as they travel around the resonator.The stretching and compression processes are implemented by dual-pass grating stretchers/compressors with internal telescopes. Autocorrelation measurements taken from outputs at various locations in the resonator (marked 1 through 4 in the figure) confirmed that the laser operated in the breathing mode, with compressed pulses as much as 40 times shorter than the longer ones.The saturable absorber imposed stable mode-locking on the laser at 1.116 GHz, the 31st harmonic of the fundamental cavity frequency, only when the pulse stretching and compression were of approximately equal magnitude. When the researchers added a 323-MHz signal to the DC bias of the gain medium, they observed mode-locking at that frequency, the ninth harmonic of the fundamental frequency. When these pulses were coupled from the laser and externally compressed with another grating pair, 274-fs pulses were observed.