A quasi-continuous-wave external cavity diamond Raman laser with 1.2 kW output power was demonstrated using gated pump pulses of 100 µs duration, which was 7 times longer than the time constant for the establishment of steady-state temperature gradients. An 83% slope efficiency and a 53% optical-to-optical efficiency were obtained in conversion from a 1.064 µm Nd:YAG pump to a 1.24 µm first Stokes. The transient Stokes behavior from the sharp turn-on was consistent with calculations for the first Stokes build-up time. A brightness enhancement factor of 56 was obtained from the M2 = 15 pump beam. An observed increase in the first Stokes beam quality from M2 = 2.95 to M2 = 1.25 with increase of the Raman laser output power indicated the presence of a steady-state thermal lens in diamond.
We demonstrate a passively mode-locked holmium-praseodymium co-doped ring fiber laser that produces an estimated 950 fs pulsewidth and peak power of 4.3 kW at a pulse repetition rate of 74 MHz. The measured center wavelength was 2.86 µm which overlaps more strongly with liquid water whilst better avoiding atmospheric water vapor which overlaps more strongly with previously reported ultrafast Er3+ fiber lasers operating at 2.8. Thus the present system should display better long term stability compared to the Er3+-based system and at the same time, be a more practical tool for interaction with biological tissues.
The laser was constructed using a 1.2 m long double-clad fluoride fiber doped with Ho3+ and Pr3+ ions and arranged into a unidirectional ring resonator that was resistant to instabilities associated with back reflections. Two semiconductor 1150 nm laser diodes with the maximum combined output of 7.5 W were used to pump the fiber. Mode-locking was achieved using the combination of two techniques: sub-picosecond pulses were produced by nonlinear polarization evolution after longer pulses were initially obtained using an in-cavity GaAs saturable absorber having a modulation depth of 90% and a relaxation time of 10 ps. A standard arrangement employing two waveplates and an optical isolator was introduced into the resonator to carry out nonlinear polarization rotation. The average power of the mode-locked laser reached 350 mW after the 50% outcoupling mirror. The RF signal-to-noise ratio reached 67 dB for the first peak at the resolution bandwidth of 10 kHz.