A stretched-pulse mode-locked laser source at a central wavelength of 1275 nm

This study presents a time-stretched wavelength-swept laser source based on stretched-pulse mode-locking. A broadband semiconductor optical amplifier (SOA) technology is used as an optical gain element. The laser comprises a unidirectional ring cavity with matched positive and negative continuously chirped fiber Bragg gratings (FBG's). One FBG generates a total positive dispersion of 454 ps/nm at 1275 nm and the other chirped FBG generates a total negative dispersion of -454 ps/nm at 1275 nm. A high-extension lithium-niobate intensity modulator (>30dB extinction at 1275 nm, 4.9 dB loss at maximum transmission) is driven with short pulses by a bit pattern generator providing approximately 0.235 ns full-width at half-maximum pulse profiles. These pulses are stretched, amplified, and compressed within the ring cavity, and the modulator pulsing is synchronized to a harmonic of the cavity round trip time. The laser output is provided from the cavity by a 25% coupler. The output light is amplified by another SOA. The laser source provides a sweeping range of approximately 90 nm centered at around 1275 nm at a repetition rate of similar to 5 MHz. This yields an estimated axial resolution of 8 mu m in air.