Coherent addition of adjacent lasers by forked eigenstate operation

Forked laser eigenstates are shown to provide a powerful tool both to phase lock spatially separated laser oscillators and to add their powers coherently into a TEM(00) output beam. Coherent addition of the powers extracted from two fiber-coupled diode-pumped Nd:YAG channels is theoretically and experimentally demonstrated. Pure TEM(00) oscillation is obtained with a 20% optical-to-optical conversion efficiency. The coherence of the two-propagation-axis laser is proved, and single-frequency operation is demonstrated. The scalability of the scheme is discussed.     

Fluid jet polishing of optical surfaces

We present a new finishing process that is capable of locally shaping and polishing optical surfaces of complex shapes. A fluid jet system is used to guide a premixed slurry at pressures less than 6 bars to the optical surface. We used a slurry comprising water and 10% #800 SiC abrasives (21.8 mum) to reduce the surface roughness of a BK7 sample from 350 to 25 nm rms and to vary the shape of a polished sample BK7, maintaining its surface roughness of 1.6 nm rms, thereby proving both the shaping and polishing possibilities of the presented method.     

Passive Q Switching of a Diode-Pumped 946-nm Nd:YAG Laser with 1.6-W Average Output Power

As much as 1.6-W average output power was emitted in a simple setup from a diffusion-bonded Nd:YAG rod with 70-100-ns Q-switched pulses at 946 nm and repetition frequencies between 15 and 45 kHz at 22-W incident diode-pump power. A Cr(4+):YAG crystal with a bleachable loss of approximately 2.5% and a length of 0.5 mm was used as a saturable absorber. The extraction efficiency was 47% in comparison with the continuous-wave laser output power of 3.37 W in the free-running regime.     

Multispectral Imaging System for the Mapping of Pigments in Works of Art by use of Principal-Component Analysis

Image spectroscopy (IS) is an important tool for the noninvasive analysis of works of art. It generates a wide sequence of multispectral images from which a reflectance spectrum for each imaged point can be recovered. In addition, digital processing techniques can be employed to divide the images into areas of similar spectral behavior. An IS system designed and developed in our laboratory is described. The methodology used to process the acquired data integrates spectral analysis with statistical image processing: in particular, the potential of principal-component analysis applied in this area is investigated. A selection of the results obtained from a sixteenth-century oil-painted panel by Luca Signorelli is also reported.     

Folded free-space polarization-controlled multistage interconnection network

We present a folded free-space polarization-controlled optical multistage interconnection network (MIN) based on a dilated bypass-exchange switch (DBS) design that uses compact polarization-selective diffractive optical elements (PDOE's). The folded MIN design has several advantages over that of the traditional transparent MIN, including compactness, spatial filtering of unwanted higher-order diffraction terms leading to an improved signal-to-noise ratio (SNR), and ease of alignment. We experimentally characterize a folded 2 x 2 switch, as well as a 4 x 4 and an 8 x 8 folded MIN that we have designed and fabricated. We fabricated an array of off-axis Fresnel lenslet PDOE's with a 30:1 SNR and used it to construct a 2 x 2 DBS with a measured SNR of 60:1. Using this PDOE array in a 4 x 4 MIN resulted in an increased SNR of 120:1, highlighting the filtering effect of the folded design.     

Laser Doppler velocimetry with a compact disc pickup

A laser Doppler velocimeter employing a compact disc pickup for both fringe projection and signal detection is described. The spectrum of the recorded signal gives the information about the speed of the object. The device takes advantage of the Talbot effect to project the grating contained in the pickup onto a moving target, so that no imaging system is required. The peculiar imaging technique allows for the exploitation of several optical configurations and permits the manipulation of the intensity profile of the projected grating. The instrument was used to measure the velocity of dust particles on a solid substrate in the 1-m/s range but could also find an application to the study of liquid flow.     

Tunable picosecond blue and ultraviolet pulses from a diode-pumped laser system seeded by a gain-switched laser diode

Picosecond pulses emitted from a gain-switched laser diode have been amplified in a Ti:sapphire regenerative amplifier indirectly pumped by a 4-W laser diode. This all-solid-state system produced microjoule pulses tunable from 803 to 840 nm at repetition rates up to 25 kHz with durations of 70-100 ps. By frequency doubling and tripling the output, we generated blue and UV pulses tunable from 401 to 420 nm and from 268 to 280 nm, respectively. Average powers larger than 4 mW were reached in these two wavelength regions.     

Neural-network method applied to the stereo image correspondence problem in three-component particle image velocimetry

The successful application of a recurrent neural network of the Hopfield type to the solution of the stereo image-pair reconciliation problem in stereoscopic particle image velocimetry (PIV) in the tracking mode is described. The results of applying the network to both virtual-flow and physical-flow PIV data sets are presented, and the usefulness of this novel approach to PIV stereo image analysis is demonstrated. A partner-particle image-pair density (PPID) parameter is defined as the average number of potential particle image-pair candidates in the search window in the second view corresponding to a single image pair in the first view. A quantitative assessment of the performance of the method is then made from groups of 100 synthetic flow images at various values of the PPID. The successful pairing of complementary image points is shown to vary from 100% at a PPID of 1 and to remain greater than 97% successful for PPID's up to 5. The application of the method to a hydraulic flow is also described, with in-line stereo images presented, and the application of the neural-matching method is demonstrated for a typical data set.     

Efficient Nd:YAG Laser Frequency Doubling with Periodically Poled KTP

Periodically poled flux-grown KTiOPO(4) was used for efficient extracavity 1064-nm Nd:YAG laser frequency doubling. A conversion efficiency exceeding 65% was obtained in Q-switched operation, and 1.34 W of average frequency-doubled power was generated with 2.2 W of mode-locked laser output. The conversion efficiency is approximately two times higher for periodically poled KTP than for conventional type II phase-matched KTP. The measurements indicate that periodically poled KTP is less susceptible to optical damage than type II KTP.     

Frequency Stability Improvement of a Two-Mode Stabilized 633-nm He-Ne Laser

The red side (lower-frequency) mode of a two-mode stabilized 633-nm He-Ne laser has been locked to the hyperfine structure of the P() line of (127)I(2) by means of frequency modulation spectroscopy enhanced by an external optical cavity. Both the red side and blue side (higher-frequency) modes of the laser exhibit a frequency stability of 2.3 x 10(-11) tau(-1/2). In addition, the frequency fluctuations of the blue side mode are detected by a Fabry-Perot cavity and compensated through an acousto-optic frequency shifter. The short-term stability of better than 3 x 10(-11) is attained for integration times of between 2 x 10(-3) and 2 x 10(-1) s.