Laser System Specifications

The DUV-FEL requires conventional laser sources to play several roles in its operation: initiating electron bunch generation, providing seed radiation, and supplying reference beams in optical diagnostics. It is important that the light sources fulfilling these various roles be synchronized with each other and with what is happening to the electron bunch in the vacuum chamber. To initiate the electron bunch, a short (~5 psec) pulse of light impinges on the photocathode of the rf gun. The low energy electrons must be produced at the rf phase at which the fields in the electron gun are optimal for the bunch acceleration. The shape and timing of this pulse are critical for assuring an energetic bunch with a low emittance. An intense tunable laser pulse must also be derived from the same source as the photocathode pulse, and coupled to the electron bunch either to seed the FEL emission, or to modulate the electron bunch energy in high-gain harmonic generation. Finally, analyzing the radiation that comes out of the machine -- both the FEL emission and weaker radiations that are useful diagnostics -- requires synchronized beams to act as references in cross-correlation and interferometric measurements.

The figure below shows a functional block diagram of the base laser system and the components used to generate and deliver the photocathode pulse. The base system produces 35 mJ of 798 nm light with a pulse width as short as 100 fsec. Only 6 mJ of this is required to generate the 266 nm light that drives the photocathode. The rest will be used to pump a tunable source that is still under construction. The system includes numerous commercial lasers pumping various other lasers, so there are a few conventions in the figure, adopted in an effort to reduce the confusion. The arrows representing optical beams are color-code for wavelength as follows:

The beams shown in dashed lines indicate weak beams (taken from a low-reflectivity beamsplitter or leakage through a mirror) used for diagnostics. Solid black lines are mirrors or beamsplitters. In the discussion that follows, the first references to labeled parts of the diagram will appear in boldface.

Starting at the lower left corner of the diagram, a Spectra-Physics Millennia CW intracavity-doubled neodymium vanadate laser produces 3.5 Watts of green light, which is used to pump a Spectra-Physics Tsunami mode-locked titanium sapphire oscillator. The oscillator produces 5 nanojoule, 100 femtosecond pulses at a wavelength of 798 nm and a repetition rate of 81.6 MHz. Diagnostics for the oscillator stage include a Spectrum Analyzer, Autocorrelator and Photodiode, which monitor the spectral width, the temporal pulse width, and the energy stability respectively. The pulse train is then passed through an Optical Isolator, which prevents optical feedback from later stages from destabilizing the oscillator.

The 100 femtosecond pulses must be stretched in time to 300 picoseconds (in the Stretcher) before being amplified, in order to prevent optical damage in the amplifiers. After amplification, the amplified pulse will be recompressed in the Compressor. Both stretcher and compressor work by taking advantage of the large spectral width inherent in short pulses. Using dispersive elements such as gratings or prisms, it is possible to make an optical arrangement that “chirps” the pulse, i.e, separates the frequency components of the pulse in time. The stretcher causes higher frequency components to come later in the pulse than the lower frequency components, while in the compressor, the sign of the chirp is reversed. By balancing the two stages, a short pulse is again obtained in the amplified, re-compressed pulse.

The three amplification stages are pumped by two Quanta Ray frequency-doubled Q-switched YAG lasers, operating at 10 Hz. The GCR-150 produces 300 mJ of 532nm light, of which approximately 40 mJ is used to pump the Regenerative Amplifier. The rest pumps 2-Pass Amplifier A. The GCR-170 supplies 450 mJ of green light to pump 2-Pass Amplifier B. It is in the regenerative amplifier that the repetition rate changes from 81.6 MHz to 10Hz. After all 3 amplification stages, the pulse energy is approximately 50 mJ. Losses in the compressor amount to approximately 30%, so 35 mJ of radiation is available for use. A small part of this is split off for diagnostics: an Autocorrelator to monitor pulse width, and Spot Imaging to monitor spatial quality.

Only 6 mJ of this light is needed to generate the 266 nm pulse that drives the photocathode. The energy is adjusted in the Power Attenuator, and the 798 nm light is frequency-doubled to a wavelength of 399 nm using type-I phase matching in a beta-barium borate (BBO) crystal (stage labeled BBO 1, 2w). The 399 nm and 798 nm light is then summed in a second type-I phase matched BBO crystal (stage labeled BBO 2, 3w) to make 266 nm light. The wavelengths are separated using dichroic mirrors (not shown). After adjusting the pulse energy (Power Atten) , the light is relayed to the photocathode, some 14 meters away. The Optical Relay images a plane near the second BBO crystal onto the photocathode, with a second image plane located nearer the entrance to the electron gun. This allows us to shape or modulate the beam using apertures or masks at either of the image planes, which is useful both for optimizing spot shape, and for generating asymmetric electron bunches for diagnostic purposes. The spot shape and position on the photocathode is monitored with a “Monument”, which is just an image plane duplicating the image on the photocathode. A weakly reflecting beamsplitter is placed just before the entrance to the electron gun, and the beamsplitter-monument separation is made equal to the beamsplitter-photocathode separation. The photocathode is also imaged at the optical beam exit from the electron gun, and a third imaging system looks at scattered light from the photocathode (these are not shown on the diagram).