As a continuing effort to develop and build state-of-the-art Ti:sapphire lasers, Q-Peak completed construction and installation of a pulsed, frequency-tripled, single-frequency Ti:sapphire laser. The system generates 40 mJ of 20-ns-pulsewidth energy in the 254-nm wavelength region. A block diagram of the system optics, which are mounted on a 2'x5' optical breadboard, appears below.
The Ti:sapphire laser is pumped by a frequency-doubled, Q-switched Nd:YLF oscillator-amplifer system, based on the compact laser head design. The Nd:YLF oscillator/amplifier has a single flashlamp pumping two laser rods, one used in the Q-switched oscillator and the other in the amplifier. The output of the oscillator/amplifer stage is split into two beams, which are input to the Nd:YLF dual amplifier stage, again, two laser rods pumped by a single flashlamp. The two beams output from the dual amplifer are frequency doubled by two KTP crystals. Each doubled beam is split in two to provide a total of four beams for pumping two, Brewster-angle Ti:sapphire crystals. The Ti:sapphire laser resonator is unstable and employs a graded-reflectivity mirror (GRM) as the output coupler. A set of Brewster-angle prisms (for coarse frequency tuning) and a Porro-prism reflector provide the rest of the resonator components. The Ti:sapphire laser output is doubled by a BBO crystal and the residual fundamental and second harmonic are mixed in a second BBO crystal to provide the third harmonic.
In order to obtain single-frequency output the pulsed Ti:sapphire laser is injection-seeded by a cw, single-frequency, ring Ti:sapphire laser, not shown in the diagram. Reliable operation on a single mode is accomplished through the use of a ramp-and-lock loop that controls the frequency of the pulsed laser resonator on a individual pulse basis through the use of a PZT-mounted fold mirror in the cavity beam path. A scanning etalon measurement indicated a FWHM linewidth of 28 MHz, close to the transform limit of 20 MHz associated with the measured pulsewidth of 22.5 ns.
The energy flow in the system is shown in the table below, representative of the best operation at 10 Hz. The system has been run at a rate of 30 Hz, but is limited at present by the properties of the crystals used to double the pump laser.
| Wavelength (nm) | Energy (mJ) |
| 1053 (Nd:YLF) | 1620 |
| 527 (2x Nd:YLF) | 830 (2 beams) |
| 760 (Ti:sapphire) | 320 |
| 380 (2x Ti:sapphire) | 172 |
| 254 (3x Ti:sapphire) | 40 |
The specific application of this system is to flow diagnostics, as carried out by the Applied Physics Group at Princeton University. Velocity, temperature and density of high-speed flows is accurately measured using the Filtered Rayleigh Scattering approach, where the filter is based on the narrow-linewidth, intense absorption in atomic mercury at 253.7 nm. Work at Princeton has been carried out by Professor R.B. Miles, and co-workers, while the Q-Peak laser development involved, among others, G.A. Rines, R.A. Schwarz and Dr. A. Finch. One reference is:
N.D. Finkelstein, W. R. Lempert, R.B. Miles, A Finch and G.A. Rines, "Cavity-locked, injection-seeded, titanium:sapphire laser and application to ultraviolet flow diagnostics," American Institute of Aeronautics and Astronautics, 34th Aerospace Sciences Meeting, Reno, NV, January 15-18, 1996 (AIAA 96-0177).
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