We have designed, built and tested an actively mode-locked fiber laser, operating at 1550 nm, for use as the sampling
waveform in an opto-electronic analog-to-digital converter (ADC). Analysis shows that, in order to digitize a 10-GHz
signal to 10 bits of resolution, the sampling pulsewidth must be less than 2.44 ps, the RMS timing jitter must be below
31.0 fs, and the RMS amplitude jitter must be below 0.195%. Fiber lasers have proven to have the capability to narrowly
exceed these operating requirements. The fiber laser is a "sigma" laser consisting of Er-doped gain medium, dispersion-compensating
fiber, nonlinear fiber, a Faraday rotation mirror, polarization-maintaining fiber and components, and diode
pump lasers. The active mode-locking is achieved by a Mach-Zehnder interferometer modulator, driven by a frequency
synthesizer operating at the desired sampling rate. A piezo-electric element is used in a feedback control loop to stabilize
the output PRF against environmental changes. Measurements of the laser output revealed the maximum nominal PRF to
be 16 GHz, the nominal pulsewidth to be 7.2 ps, and the nominal RNS timing jitter to be 386 fs. Incorporating this laser
into a sampling ADC would allow us to sample a 805-MHz bandwidth signal to a resolution of 10 bits as limited by
timing jitter. Techniques to reduce the timing-jitter bottleneck are discussed.
Solid-propellant rocket fuels frequently have spherical aluminum pellets added to improve combustion efficiency and to improve thrust performance. In order to model the combustion process, information is required about the size of the particles as they lift off of the propellant surface. We have used optical pulsed-laser holography to record an approximate 2.5 cm X 2.5 cm X 2.5 cm volume at the propellant surface in a test rocket motor. A pulsed ruby laser, combined with a laser line filter to remove the flame light, records the hologram. A diffuser is used to remove the phase gradients introduced by the thermal effects in the flame. The scene is reconstructed with a krypton laser, viewed with an optical microscope, and captured on videotape. The recorded scene is digitized and analyzed with digital image processing techniques on a personal computer equipped with a video memory board and an image processing subroutine library. The image processing techniques developed reduce the speckle in the scene, apply a threshold to differentiate the particles from the background, and locate and size the particles. Statistical analysis of the sizing results provides a histogram representation of the particles size distribution. Particle resolution down to 10 micrometers has been achieved with this technique.