We report on a passively mode-locked, diode-pumped femtosecond laser that provides ~200fs, 1053nm optical solitonlike pulses with < 8 orders of magnitude (OM) temporal contrast. The average output power of the compact, air-cooled laser is ~450mW at 77MHz pulse repetition rate. The spectral bandwidth (FWHM) is around 5-6nm, corresponding to transform limited pulses. For synchronization purposes the pulse repetition rate can be fine tuned to and continuously kept at a desired value employing a controller circuit, a motorized translation stage and a piezo actuator. The center wavelength of the pulses is tunable and can be precisely set to 1053nm by adjusting the laser pump power. The selfstarting laser is mode-locked using a semiconductor saturable absorber mirror (SESAM). The SESAM parameters are optimized for producing fs pulses with high temporal contrast. The effects of an intra-cavity fused silica plate inserted near Brewster angle and used for fine wavelength tuning and for polarization selection on the temporal contrast of the emitted pulses are discussed. The high temporal contrast pulses from this laser oscillator are well suited for seeding very high gain, multiple Joule type, Nd:glass, chirped pulse amplification systems routinely used in high intensity laser interaction experiments. The automated high dynamic range autocorrelator (HDR-AC) capable of recording the autocorrelation trace over 8 OM is described. The HDR-AC is based on a BBO crystal for second harmonic generation and a PMT for high sensitivity detection. A lock-in amplifier increases the detectable signal range by ~2 OM.
Commercial picosecond sources have found widespread applications. Typical system parameters are pulse widths below 20 ps, repetition rates between 0.1 and 2 MHz, and microjoule level pulse energies. Most systems are based on short pulse mode-locked oscillators, regenerative amplifiers, and pockel cells as active beam switches. In contrast, we present a completely passive system, consisting of a passively Q-switched microchip laser, a single-stage amplifier, and a pulse compressor. The Q-switched microchip laser has a 50-μm-long Nd:YVO4 gain material optically bonded to a 4.6-mm-thick undoped YVO4 crystal. It delivers pulse widths of 40 ps and repetition rates of 0.2 to 1.4 MHz at a wavelength of 1.064 μm. The pulse energy is a few nanojoule. These 40-ps pulses are spectrally broadened in a standard single-mode fiber and then compressed in a 24-mm-long chirped Bragg grating to as low as 3.3 ps. The repetition rate can be tuned from ∼0.2 to 1.4 MHz by changing the pump power, while the pulse width and the pulse energy from the microchip laser are unchanged. The spectral broadening in the fiber is observed throughout the pulse repetition rate, supporting sub-10-ps pulses. Finally, the pulses are amplified in a single-stage Nd:YVO4 amplifier up to the microjoule level (up to 4 μJ pulse energy). As a result, the system delivers sub-10-ps pulses at a microjoule level with about 1 MHz repetition rate, and thus fulfills the requirements for ps-micromachining. It does not contain any active switching elements and can be integrated in a very compact setup.
Commercial picosecond sources have found widespread applications. Typical system parameters are pulse widths below 20 ps, repetition rates between 0.1 to 2 MHz, and micro Joule level pulse energies. Most systems are based on short pulse modelocked oscillators, regenerative amplifiers, and pockel cells as active beam switches. In contrast we present a completely passive system, consisting of a passively Q-switched microchip laser, a single-stage amplifier, and a pulse compressor. The Q-switched microchip laser has a 50 μm long Nd:YVO4-gain material optically bonded to a 4.6 mm thick undoped YVO4-crystal. It delivers pulse widths of 40 ps and repetition rates of 0.2 – 1.4 MHz at a wavelength of 1.064 μm. The pulse energy is a few nJ. These 40-ps pulses are spectrally broadened in a standard single mode fibre and then compressed in a 24 mm long chirped Bragg grating to as low as 3.3 ps. The repetition rate can be tuned from app. 0.2 to 1.4 MHz by changing the pump power while the pulse width and the pulse energy from the microchip laser are unchanged. The spectral broadening in the fibre is observed throughout the pulse repetition rate, supporting sub-10- ps pulses. Finally, the pulses are amplified in a single-stage Nd:YVO4-amplifier up to the microjoule level (up to 4 μJ pulse energy). As a result the system delivers sub-10-ps pulses at a microjoule level with about 1 MHz repetition rate, and thus fulfills the requirements for ps-micromachining. It does not contain any active switching elements and can be integrated in a very compact setup.
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