Ultrafast lasers can be used to produce laser pulses with enormous peak powers and power densities. The very high peak power that can be achieved with femtosecond pulses means that in principle, nonlinear frequency conversion should be very efficient. It should be quite straightforward to use second-harmonic (SHG), third-harmonic (THG) and fourth-harmonic generation (FHG) to produce femtosecond pulses in the near- to deep-ultraviolet. We present results on a mode-locked Yb3+-fiber laser operating in the 980 nm spectral band. Such lasers are very attractive as a seed source for generating blue light using SHG. The laser comprised a linear fiber cavity defined by the fiber loop-mirror and the semiconductor saturable-absorber mirror (SESAM) used to self-start the mode-locking. SESAM operating in the 940-1050 nm wavelength-range comprised 26 pairs of AlAs/GaAs quarter-wave layers that form a distributed Bragg reflector with a center wavelength at about 1000 nm. The active region consists of five GaInNAs quantum wells embedded within GaAs layers. With proper alignment of the laser cavity, the laser was self-starting for pump powers above 50 mW at 915 nm. The output mode-locked pulse train at about 980 nm had an output power of 3 mW, a repetition rate of 30 MHz and pulse duration of 2.3 ps. The pulse spectrum exhibited soliton sidebands at all pump powers, confirming that the laser operates in the anomalous-dispersion regime. The time-bandwidth product was equal to 0.47, indicating that the pulses were nearly bandwidth-limited with Gaussian temporal and spectral profiles. The average value of the cavity dispersion near 1 µm, estimated from the soliton sidebands, was -1.6 ps2. With a master oscillator power amplifier configuration (MOPA) more than 200 mW of the output power is expected with just two single-mode pump laser diodes.
We demonstrate a practical ultra-fast Nd-doped fiber laser operating in the 894-909 nm spectral range, in both soliton and stretched pulse dispersion supporting regimes. Using purposely designed semiconductor saturable absorbers, a truly self-started mode-locking regime of operation with clean, transform limited pulses, was achieved.
Before processing the InGaAsN/GaAs edge emitting lasers, post-growth rapid thermal annealing (RTA) was applied on the wafer. Different RTA results in different threshold current density (Jth). RTA at 720°C reduces the Jth significantly but keeps the linear fit slope of Jth vs 1/L (L is the cavity length). It indicates that RTA at 720°C can decrease the absorption losses. High temperature RTA at 890°C can dramatically decrease the linear fit slope, which indicates that the carrier conductivity is improved dramatically even the RTA time is only one second.