System Description:
Design changes and compromises (to allow the system to be used for NLC Source , LCLS, and NLC laser wire)
For NLC, the mode locked seed laser will need to have an extra brewster tuner plate to reduce its bandwidth.
For the LCLS the repetition rate control will need to select a single pulse, for the other systems it will need to select a train of pulses.
For the LCLS, the ML oscillator could have a longer cavity an lower repetition rate. This would simplify the design of the pulse selection system.
For the NLC laser the long pulse YAG could operate at about 40% of the power required for the LCLS. The laser wire could operate at lower power, but the signal to noise improves as the power is increased.
Sub-system Designs
CW Pump Laser: These are available commercially from several vendors.
Mode Locked Ti:Sapphire Oscillator: For the NLC and Laser Wire this must be a very short cavity oscillator to allow operate at 714Mhz. Conventional mode-locked Ti:Sapphire oscillators use a pair of prisms to compensate for the material dispersion in the Ti:Sapphire crystal. This system will use "chirped" mirrors which have a built-in dispersion to compensate for the Ti:Sapphire crystal dispersion. Chirped mirrors are a relatively new technology, but are becoming more widely used.
Mode locking is obtained through the non-linear Keer effect in the Ti:Sapphire crystal. Higher peak powers increase the non-linear focussing in the crystal, resulting in a smaller mode size and better coupling to the pump beam. The result is that high peak powers are enhanced, resulting in passive mode-locking.
The LCLS and laser wire require very low timing jitter (<50fsec RMS), considerably better than current state of the art of 200fsec. Typically a mode-locked laser of this type is phase locked to an external RF source by adjusting the cavity length (and therefore the round trip time) with a piezo-electric actuator on one mirror. This works, but has the disadvantage of requiring the loop bandwidth to be less than about one kilohertz.
We propose modulating the input pump power (which can be done on microsecond time scales), which will change the circulating power, and through the Keer effect, change the effective cavity length. This should allow much high bandwidths, and lower timing jitter. Note that this is required for the LCLS and Laser Wire, but not for the NLC source laser.
Pulse Stretcher and Shaper: A conventional two grating stretcher is used. This provides a location where the spatial profile of the beam is correlated with wavelength. A spatial light intensity and phase modulator inserted here can control the spectrum of the optical pulse. For the LCLS and Laser Wire, this is used to control the Fourier transform of the pulse before compression. For the NLC source laser, the pulse is not re-compressed, and the spectral shape is directly translated through dispersion to the output temporal shape.
Fiber Stretcher: A grating stretcher cannot easily provide the required dispersion to produce long, narrow bandwidth pulses for the NLC source laser. An approximately 5Km fiber is used to provide dispersion. The pulse has already been stretched to about 10psec so non-linear effects should not be too severe.
Repetition Rate Control: This is performed with a Pockels cell and high voltage pulse generator. FET pulsers can provide the long pulse train required for NLC and Laser Wire. Avalanche transistor pulsers can slice a single pulse for LCLS. Note that if arbitrary selection of pulses is required for NLC, some pulser development will be needed.
Multi-pass Ti:Sapphire Amplifier: This amplifier uses multiple passes (probably a total of about 10) to provide a gain of between 10^5 and 10^7. This amplifier is conventional in design, but due to the low input energy, considerable care must be taken to eliminate amplified spontaneous radiation.
Intensity Control:A standard Pockels cell polarizer combination
Pulse Compression: A conventional two grating pulse compressor. Not used for the NLC source laser.
Frequency Tripler (Ti:Sapphire beam): Conventional, probably BBO.
Long Pulse YAG pump laser: Usually a conventional flash-lamp pumped, Q-switched, frequency doubled YAG (producing a 5nsec pulse) is used for this application. We are proposing to instead use a very long pulse (1usec) laser. This long pulse system will have the following advantages:
1. The long pulse allows feedback to stabilize the YAG intensity while observing the florescence of the Ti:Sapphire crystals. This should provide much better intensity stability than the typically 2%RMS from conventional YAG lasers.
2. The long pulse will allow much higher pump energy densities (factor of square root of pulse length or >10). This will allow the use of fewer passes in the Ti:Sapphire amplifier, or a better safety factor with respect to damage thresholds.
3. Additional intensity stabilization methods are available - such as shutting off the YAG pump when the integrated energy on the Ti:Sapphire crystal has reached the required level.
Long Pulse YAG Pump Laser Components
CW Nd:YAG laser: Commercial, available from several vendors.
Pulse Shape and Intensity Control: Conventional, using a Pockels Cell and function generator.
Long Pulse YAG Amplifier: Uses a series of laser rods with interstage spatial filtering and temporal slicing to reduce amplified spontaneous emission. Uses birefringence compensation. This system uses 4mm diameter rods for the pre-amplifier stages, and 6mm rods for the power amplifiers.
Frequency Doubler: This is one of the most difficult parts of the pump system.The narrow output linewidth of the laser (determined by the seed laser) may allow efficient doubling in KDP. Detailed simulations are required to determine the feasibility of this approach.
Research and Development Required
1. The short cavity, chirp mirror Ti:Sapphire oscillator is expected to work, but must be demonstrated
2. The timing stabilization system (required for LCLS and Laser Wire) requires performance which exceeds the current state of the art. The intensity control of phase system must be demonstrated.
3. The long pulse YAG laser amplifier chain must be demonstrated.
4. The long pulse YAG frequency doubler must be demonstrated
5. The Ti:Sapphire amplifier system is expected to function, but is near the limits set by amplified spontaneous emission.
Page by Josef Frisch frisch@slac.stanford.edu 04/22/2002