Mode locking by a frequency-doubling crystal: generation of transform-limited ultrashort light pulses

Nonlinear-mirror mode-locked crystal waveguide laser by intracavity fourth-harmonic loss modulation

We present a nonlinear-mirror (NLM) mode-locked crystal waveguide laser. By adding nonlinear crystals into traditional NLM devices, the fourth harmonic is generated to form loss modulation, which suppresses the Q-switching instability of mode-locked lasers and achieves the optimal equivalent transmittance. The NLM mode-locked laser delivers ∼30 W average power with a repetition rate of 32.2 MHz and a pulse width of 950 fs. It is revealed that this novel, to the best of our knowledge, design with simple, robust, and reliable structure has a great potential in the development of high-power mode-locked laser.

Self-mode-locking through intra-cavity sum-frequency generation

A new technique for mode-locking is demonstrated based on two lasers sharing one leg for sum-frequency generation. When the two lasers had equal round trip time one will produce bright pulses and the other dark pulses. Both lasers used Nd:YVO₄ as the gain material, but operated at different wavelengths, namely 1064 nm and 1342 nm. In the present configuration, sub-250 ps pulses were generated at a repetition rate of 276 MHz with an output power of 70 mW. With appropriate choice of round trip loss at the two wavelengths it was possible to choose which laser was generating the bright pulses.

Observation of strong cascaded Kerr-lens dynamics in an optimally-coupled cw intracavity frequency-doubled Nd:YLF ring laser

Self-starting self-pulsing dynamics at the cavity free-spectral-range frequency were observed in intracavity second-harmonic generation of a diode end-pumped Nd:YLF ring laser containing a periodically-poled KTiOPO(4) (ppKTP) nonlinear crystal. Although the unidirectional laser was designed for continuous-wave (cw) single-frequency operation, with a resonator set at the middle of its stability range, partial Kerr-lens mode-locking (KLM) arose spontaneously once the ppKTP was inserted. This ultrafast dynamics along with a strong spectral gain broadening, not observed with any birefringent nonlinear doubler, is associated to the finite bandwidth of the quasi-phase-matched crystal with respect to the laser gain bandwidth, leading to giant cascaded Kerr-lensing effects when the ppKTP temperature is detuned from perfect quasi-phase-matching either in the self-focusing or defocusing sides. While under partial KLM operation the laser delivered only approximately 0.14W of broadband red output power, single-frequency operation could be only achieved by using an intracavity etalon with a suitable partial reflectivity (R>or=25%), leading to an optimally (approximately100% efficiency) out-coupled 1.4 W red power at 660.5 nm, as much as the fundamental 1321 nm power that could be extracted from the unidirectional laser using an optimal T = 2% output coupler.

Mode locking using stimulated Raman scattering

Recent experiments have shown that the simple addition of a length of Raman-shifting fibre in the cavity of a cw-pumped fibre laser can cause the laser to generate a stable train of pulses [Zhao and Jackson, Opt. Lett., 31 751 (2006)]. We show using a numerical model that this behavior is a new type of mode locking, driven by backward stimulated Raman scattering. This mode locking mechanism could also be applied to crystalline Raman laser systems to create a novel picosecond oscillator.

Two-dimensional optical spatiotemporal solitons in quadratic media

Numerical and experimental studies of the propagation of femtosecond-duration optical pulses in quadratic nonlinear media are presented. Pulse evolution is investigated over wide ranges of initial intensity and phase mismatch between fundamental and harmonic waves, and the conditions that produce two-dimensional spatiotemporal solitons are delineated. Spatiotemporal solitons can be generated when the group velocities of the fundamental and harmonic fields are quite different, for proper choice of the phase mismatch. The factors that limit the formation of spatiotemporal solitons are discussed.

Femtosecond Kerr-lens mode locking with negative nonlinear phase shifts

We report a Kerr-lens mode-locked Cr:forsterite laser operated with negative nonlinear phase shift. The nonlinear phase shift is induced by the cascade chi((2;)):chi((2)) process in a lithium triborate crystal. Employing the cascade process at large phase mismatch produces a nearly linear frequency chirp. Transform-limited pulses as short as 60 fs are generated with positive cavity dispersion.

Tunable visible solid-state lasers based on second-harmonic generation of LiF:F(2) in potassium titanyl phosphate

A new broadly tunable visible solid-state laser is reported. Wavelengths between 550 and 610 nm are generated by intracavity frequency doubling of tuned and free-running room-temperature pulsed LiF:F(2)(-) lasers in potassium titanyl phosphate. Second-harmonic energy of 1.3 mJ has been achieved, corresponding to a fundamental-to-second-harmonic conversion efficiency of 20%. Operation is optimized with respect to LiF:F(2)(-) laser parameters.

High-power diode-pumped picosecond Nd(3+):YVO(4) laser

A diode-pumped Nd(3+):YVO(4) laser passively mode locked by a nonlinear mirror based on second-harmonic generation has been developed. Pumping with a fiber-coupled 10-W diode array, we generated nearly diffraction limited, linearly polarized 11-ps pulses at 1064nm, with average power of 2.7 W at a 150-MHz repetition rate. Pulses as short as 7.9 ps were measured with reduced output power (1.35 W) by optimal focusing in the nonlinear crystal.

Nonreciprocal optical device based on second-harmonic generation

We propose a novel device that exhibits nonreciprocity in optical reflection or transmission. The device is formed by two frequency-doubling crystals and a suitable dichroic mirror or a filter. The nonreciprocity may exceed 100 and be accompanied by nonlinear reflection or transmission. Applications in unidirectional and mode-locked lasers are discussed.

Frequency-doubling mode locker: the influence of group-velocity mismatch

The influence of the group-velocity mismatch between the fundamental and the second harmonic in the frequency-doubling nonlinear mirror is investigated theoretically. The nonstationary analysis provides an estimation for the maximum pulse-shortening capabilities of the device when it is used as a passive mode locker. The pulse-shape deformation is also investigated.

Frequency-domain analysis of the mode-locking process in a laser with a second-harmonic nonlinear mirror

We present a computer simulation of the mode-locking process in a pulsed Nd:YAG laser with a nonlinear mirror based on second-harmonic generation. For the first time to our knowledge, a frequency-domain description of the passive mode-locking process is presented, which provides complete information for the amplitude and phase evolution of the individual modes. The mode locking is described as being due to the interaction of the longitudinal modes in the nonlinear mirror through second-harmonic and sum- and difference-frequency generation.

Mode locking of a Nd:YAlO(3) laser at the 1.34-microm transition by a second-harmonic nonlinear mirror

A second-harmonic nonlinear mirror formed by a LiIO(3) frequency doubler and a dichroic output mirror is employed to mode lock the 1.34-microm transition of a pulsed Nd:YAlO(3) laser. Pulses of 15-ps duration are reliably generated simultaneously at 1.34 and 0.67 microm.

Passive mode locking of a cw Nd:YLF laser with a nonlinear external coupled cavity

Using an optical fiber as the nonlinear medium in an external cavity, we demonstrated passive mode locking of a cw Nd:YLF laser at a 1.053-microm wavelength. Nearly transform-limited pulses as short as 3.7 psec were obtained at a repetition rate of 76 MHz directly from the laser output without further compression. The average output power ranged from 1 to 7 W.