Oscillator
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Oscillator Physics: The oscillator is pumped continuously with 527 nm green light. A Ti:Sapphire crystal is used as the lasing medium and emits a range of wavelengths that superimpose to create an ultra-short mode-locked pulse. Due to the high intensities in the crystal, a Kerr lens affects the laser cavity. The pulse then travels through a set of prisms to compensate for the dispersion in the Ti:Sapphire crystal. These prisms assist in mode-locking the oscillator by changing the phases of the various wavelengths of the pulse. Sometimes the prisms need to be slightly adjusted to maintain mode-locking. The pulse then travels back through the crystal gaining more energy. Since the pulse intensity is much greater than the continuous beam it is more focused by the Kerr lens can be discriminated by an intracavity hard aperture which primarily blocks continuous beam. The final result is a laser that creates an ultrafast short pulse train that seeds the laser system. Oscillator Design: The oscillator’s 2.2 mm long Ti:Sapphire crystal is cut at Brewster’s angle and is cooled to 18°C to prevent thermal lensing and damage to the crystal. The construction uses a folded cavity with fused silica prisms. The optimal pulse dispersion compensation with no power loss occurs when the beam enters both prisms 3 mm from their tips. A slight blue tuning occurs in the prisms to assist with mode-locking. Sometimes small adjustments need to be made in the second prism and the hard aperture (normal width marking of ~0.9) to maintain mode-locking. When operating with CW and the aperture fully open, the power is typically 160 mW for 2.2 Watts from the 527 nm green pump. The mode-locked pulse train serves as reference timing signal and the seed pulse for the laser system. The mode-locked pulses are 20 femtoseconds at 76 MHz, with a center wavelength at 800 nm and a full width half max (fwhm) of 70 nm. They have an average power of 210 mW when pumped with 2.2 Watts of 527nm green light. |
Stretcher
Stretcher: The purpose of the stretcher is to expand the temporal length of the pulses, decreasing their peak power allowing them to travel through the optics and crystals without damaging them. The stretcher consists of two 1200 groove/mm gratings in parallel configuration, a large concave mirror and a retro-reflector. The pulse enters the chamber and hits the first grating causing the different wavelengths of light to be reflected at different angles. The pulse is then reflected off of a series of mirrors to the second grating. From here the pulse travels to the retro-reflector which changes the height of the beam in the chamber. The pulse then travels back through the grating and mirror setup but can now exit the chamber due to the retro-reflector. A diode measures the pulse train to provide the synchronization clock for the laser. This synchronization signal is sent to the Pockels cell driver which will inject one of the stretched pulses into the regenerative amplifier (regen). After the pulse leaves the stretcher it goes through a faraday isolator to prevent feedback from the regen to the oscillator. The result is the 20 femtosecond pulses coming out of the oscillator are stretched to about 400 picoseconds due to the difference in optical path length of the various colors. | 
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Regen
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Regen: The regen has a ring cavity design that uses two Ti:Sapphire rods with a 22 mm effective path as the gain medium. The beam enters the regen, is reflected off of a thin film polarizer and goes through a half wave plate which rotates the polarization of the light. Injection seed pulses from the oscillator trigger the Pockels cell (8 Volts) which acts as another half wave plate effectively trapping the pulse in the cavity for 17-18 round trips. The regen cavity is pumped by the green light from a Surelite laser with 0.2 Watts at 10 Hz. This Surelite laser is divided once it enters the cavity to pump both crystals equally from each side to ensure that the pulse is properly amplified. The pulse is amplified from a few hundred picojoules to 5 mJ at a much slower repetition rate of 10 Hz. Once the pulse is sufficiently amplified, the Pockels cell switches back to 0 V allowing the pulse exit the cavity via the thin film polarizer. In the regen cavity TEM00 gain is strongly favored and consequently gain narrowing occurs reducing the bandwidth to 30 nm. A transmission etalon counteracts this effect by decreasing the intensity around 795 nm, restoring pulse bandwidth to 55 nm. |
Multipass
Multipass: The multipass amplifier is designed in a bowtie shape that passes the beam through the gain medium a total of 5 times. The gain medium is a Ti:Sapphire crystal 15 mm in diameter and 21 mm in length which is pumped equally from both sides by a Quanta Ray Nd:YAG laser from Spectra Physics at 10 W. The crystal is mounted on a copper stage with 20°C circulating water to extract heat, counteracting thermal-lensing. Micro-lenses are implemented to refine the pump lasers to the best possible mode quality. As a result, the multipass amplifier can amplify 6 mJ pulses to 290 mJ at 10 Hz. |  |
Compressor
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Compressor: The compressor is designed to be the exact opposite of the stretcher, making the pulses shorter to increase their peak power. The compressor can also be adjusted to account for any dispersion in the laser system. It consists of two 1500 groove/mm gratings mounted in parallel with a center to center separation of 40 cm. The beam is expanded to a 2 cm FWHM diameter before entering the compressor to prevent air breakdown and damage to the optics. Due to the efficiency of the gratings and other factors the compressor has a throughput of 67%. Once focused, the final result is a 40-45 femtosecond pulse with 6 Trillion Watts of energy. |
