Picosecond Lasers

Today, diode-pumped cw Nd :YAG or Nd : YLF lasers, mode-locked with an acousto-optic modulator, are the standard sources for the production of picosecond pulses. A typically end-pumped and actively mode-locked neodymium laser is shown in Fig. 1. The output from a GaAlAs diode array is focused onto the end of the laser crystal. Spot sizes of the pump beam range from 50μm to a few hundred micrometers. A common resonator configuration for diode-pumped actively mode-locked lasers is a three-mirror arrangement. A folded cavity geometry provides a beam waist at both the laser crystal and the modulator, as well as astigmatic compensation. In commercial mode-locked lasers, the resonator is usually folded a few times to decrease the overall length of the system. The length of the cavity is usually a compromise between the need for short pulses, which requires a short resonator and a high-modulation frequency, and the ability to slice out a single pulse with a Pockels cell.
Typical of end-pumped lasers is the highly reflective coating applied to the back face of the crystal (Fig. 1) to form one of the resonator mirrors. Small thermal or vibrational changes in the pump radiation readily induce relaxation

FIGURE 1. Typical end-pumped and actively mode-locked neodymium laser.

oscillations in mode-locked systems. These relaxation oscillations cause fluctuations in amplitude, accompanied by damped oscillations at frequencies in the 30 to 150 kHz range. Therefore, careful alignment and isolation is required for clean mode-locking. The modulator is usually placed close to the front mirror at the beam waist created by the folding mirror. The resonator length is typically on the order of 1m or longer to provide a sufficiently large separation between pulses for pulse selection by an external switching device. Actively mode-locked lasers are very sensitive to cavity length detuning. In practice, cavity length changes on the order of 1μm can cause serious degradation in pulse quality. Therefore, active length stabilization of the resonator by a feedback loop is needed to maintain long-term stability of mode-locking.
Active mode-locking of a laser can be achieved by using a tunable rf oscillator and adjusting the modulator frequency to agree with the cavity length; or, alternatively, selecting a fixed frequency and adjusting the mirror spacing.