Seeded solid-state laser bolsters

From two-dimensional flow dynamics to real-time velocity profiles, particle image velocimetry (PIV) helps discern complex fluid flows in applications ranging from aerospace engineering to automotive-engine and body design. Using lasers to illuminate tiny particles, PIV typically captures particle movement with high-resolution charge-coupled device (CCD) cameras. The resulting images provide multiple-point velocity measurements of flow fields.

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Seeded solid-state laser bolsters

particle image velocimetry

From two-dimensional flow dynamics to real-time velocity profiles, particle image velocimetry (PIV) helps discern complex fluid flows in applications ranging from aerospace engineering to automotive-engine and body design. Using lasers to illuminate tiny particles, PIV typically captures particle movement with high-resolution charge-coupled device (CCD) cameras. The resulting images provide multiple-point velocity measurements of flow fields.

Many types of lasers are used for PIV flow visualization, the most popular being the Nd:YAG laser. One of the primary advantages of pulsed Nd:YAG lasers is that they have two separate resonators. This allows users to vary pulses virtually infinitely, making it possible to conduct slow-flow studies of less than

1 m/s and supersonic flow studies of greater than 1000 m/s. Another important characteristic is nanosecond pulses, an attribute that permits measurement of highly turbulent flows. Additionally, pulsed Nd:YAG lasers offer high-energy pulses, resulting in large fields of view and excellent spatial resolution in confined flows.

Recent developments in injection-seeded Nd:YAG lasers are enabling new applications in holographic PIV (HPIV) and molecular filter-based diagnostic techniques such as filtered Rayleigh scattering (FRS), global Doppler velocimetry (GDV), and planar Doppler velocimetry (PDV). The Quanta-Ray PIV Series from Spectra-Physics Lasers (Mountain View, CA) is designed with these applications in mind. Introduced at the 1997 Conference on Lasers and Electro-Optics in Baltimore, MD, these injection-seeded lasers produce two independent, 532-nm pulses with energies up to 400 mJ and coherence lengths exceeding 2 m. They also feature broad frequency tunability (>30 GHz) and precise spatial-beam overlap.

To record clear HPIV images and measure velocities using PDV, long coherence length and narrow linewidth are imperative. For HPIV, required laser bandwidth typically falls in the 150-MHz range. In PDV applications, it`s important to have a narrow linewidth that can be accurately tuned to the edge of an absorption line (iodine, for example). The flow velocity induces a Doppler shift in the scattered light that affects transmission through the iodine cell and thus to the CCD camera. This technique provides an abundance of velocity and thermodynamic details about the measured flow field.

By nature, conventional pulsed Nd:YAG lasers possess broad bandwidths of about 50 GHz, providing only a few centimeters of coherence length. To achieve linewidths of 120 MHz--equivalent to a coherence length of about 2 m-- SPL`s PIV lasers are injection seeded with a diode-pumped, single-frequency Nd:YVO4 laser. When the PIV oscillators are Q-switched, selected axial modes build up from the seeded signal rather than randomly from noise. Using this technique ensures that identical single-frequency performance is achieved in both of the seeded cavities.

Injection seeding

The seed laser is also temperature tunable over the gain bandwidth of the Nd:YAG laser and can be locked to a known reference frequency by providing a feedback signal. To match the seeder frequency, piezoelectrics in the PIV oscillators control the resonator length of each cavity. To eliminate the system`s dependence on environmental temperature, each resonator is composed of three composite graphite bars mounted on a low-expansion aluminum alloy structure. The graphite rods offer excellent length stability and, together with the surrounding structure, afford torsional stiffness and dampen vibrational effects.

To produce uniform laser light sheets for flow illumination it is essential to have excellent spatial-mode quality (see Fig. 1). Spatial mode is affected most by thermal stresses induced in the Nd:YAG rods themselves. To correct this unavoidable problem, these PIV systems feature dual-rod oscillators, each compensating for the negative effects of the other. Large, gold-coated elliptical pump-chamber reflectors produce even Nd:YAG rod illumination from a single flashlamp, while proprietary optical components allow high-energy output with low beam divergence and good beam propagation.

Because PIV applications can be diverse, Quanta-Ray PIV lasers are engineered to be flexible. For polarization image separation, fluorescence seeding, and light-scattering techniques, these lasers can be configured with second-, third-, and fourth-harmonic-generation schemes with consecutive pulses polarized either parallel or orthogonal to each other. Further, the PIV lasers can be configured with a double-pulse option producing a total of four consecutive pulses and with repetition rates of up to 30 Hz. The oscillators provide pulse-separation times ranging from 100 ns to more than 100 ms, while maintaining constant energy output and good beam characteristics. Dual output ports allow each oscillator to operate independently when necessary.

To enhance system reliability, all beam-combining mounts and optics are integrated in a single head design (see Fig. 2). Consequently, the systems offer excellent long-term beam overlap and pointing stability. All frequency-doubling and harmonic-generation accessories used to convert the Nd:YAG laser beam to usable wavelengths also are fully integrated within the laser head. This design approach overcomes alignment and stability problems that occur when components are mounted on a separate structure.

The lasers feature a compact, single-phase, air- or water-cooled power supply with remote control and TTL analog or optional IEEE or RS-232 computer interfaces. This helps ensure reliable performance in demanding environments such as large wind tunnels, where downtime can be extremely expensive. The power supply consists of an all-digital control system, two independent pulse-forming networks, and switching power supplies designed to meet stringent CE and VDE regulatory requirements.

Enabling new applications

Research efforts currently are underway in government and university laboratories aimed at the development of molecular filtered-based diagnostic techniques. Gregory Elliott and Nick Glumac of Rutgers University (New Brunswick, NJ) and Campbell Carter of Innovative Scientific Solutions Inc. (Dayton, OH) recently have incorporated PDV-related FRS in combustion environments to measure instantaneous, two-dimensional temperature fields. A benefit of FRS is its ability to make temperature measurements near solid boundaries where other laser-based techniques encounter difficulty.

Using PDV, Elliott and Carter have made velocity measurements in large wind tunnels with experimental uncertainties of less than 2 m/s. With Steve Arnette of Sverdrup Technology (Dayton, OH), they have also developed a new molecular filtered-based technique termed two-color planar Doppler velocimetry. In this work, green and red illuminations are used to acquire instantaneous velocity measurements in a Mach 1.5 axisymmetric jet. Filtered, two-color images provide real-time velocity-field representations without image splitting or postprocessing realignment.

In another application, Kansas State University assistant professor Hui Meng leads a team trying to develop a new holographic PIV technique, which offers complete 3-D information, as opposed to single-plane data provided with conventional PIV. Funded by the National Science Foundation, Meng relies on highly coherent, single-frequency output to record 3-D time and space data. Meng`s approach consists of two stages: recording interference fringe patterns on a hologram and using a CCD camera to collect 3-D flow volumes. HPIV enables the acquisition of velocity fields with millions of points in each dimension.

Meng`s HPIV technique promises to aid air- and fuel-flow analyses in internal combustion engines, aerodynamics studies for cars, airplanes and other vehicles, and the understanding of how pollutants dissipate in the atmosphere, just to name a few. By predicting and evaluating flows, HPIV offers a tool that can help engineers acquire data to validate their computational models and ultimately, improve processes and designs. o

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FIGURE 1. Spatial-mode quality of PIV laser permits uniform illumination with light sheets. Image shows vortex contour on selected planes.

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FIGURE 2. Injection-seeded PIV laser produces two independent 532-nm pulses.

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