Xiaoyuan Peng

Studies of digital microscopic holography with applications to microstructure testing

We propose an in-line digital microscopic holography system for testing of microstructures. With the incorporation of a long-distance microscope with digital holography, the system is capable of imaging test microstructures with high resolution at sufficient working distances to permit good illumination of samples. The system, which was developed in an in-line configuration, achieves high imaging capacity and exhibits properties that are favorable for micromeasurement. We demonstrate the performance of the system with experiments to determine the displacement of a silicon microcantilever and with investigations of the microscopic resolution capability.

Imaging analysis of digital holography

In this study we focus on understanding the system imaging mechanisms given rise to the unique characteristic of discretization in digital holography. Imaging analysis with respect to the system geometries is investigated and the corresponding requirements for reliable holographic imaging are specified. In addition, the imaging capacity of a digital holographic system is analyzed in terms of space-bandwidth product. The impacts due to the discrete features of the CCD sensor that are characterized by the amount of sensitive pixels and the pixel dimension are quantified. The analysis demonstrates the favorable properties of an in-line system arrangement in both the effective field of view and imaging resolution.

Study of the mechanical properties of Nd:YVO 4 crystal by use of laser interferometry and finite-element analysis

A hybrid method that combines experimental and numerical approaches for determining the material properties of Nd:YVO4 is reported. In the experimental investigations a laser interferometer is proposed for measuring the physical deformation of lasing materials at the end-pump surface. By matching with the measured end bulging, we have implemented a numerical solution with finite-element analyses to determine the Poisson ratio and Young’s modulus of the crystal. The accuracy interval of the evaluated Poisson ratio of 0.33 and Young’s modulus of 133 GPa is discussed numerically. Based on the mechanical properties obtained, the end effect is separated from thermal effects, and it shows that the end effect results in an approximate equal thermal lensing effect compared with the index parts for end-pumped Nd:YVO4 lasers.

Power scaling of diode-pumped Nd:YVO/sub 4/ lasers

The evaluation of pump capability and scaling the output power of Nd:YVO/sub 4/ (vanadate) lasers are presented in this paper. Taking into account thermal fracture limitation and fundamental mode operation, systematic investigations of vanadate crystals have been conducted in an effort to scale the output power of diode-pumped lasers to higher levels. Based on the limitation of pump power, the input versus output power and beam propagation factor of vanadate lasers are optimized with knowledge of the thermal lensing effect and maximum pump power. The theoretical analyzes provide a good prediction for the output power of diode-pumped Nd:YVO/sub 4/ lasers, which is in agreement with the experiment. A practical example of a single-end-pumped vanadate laser is demonstrated with continuous-wave output powers of 9.8 W in the TEM/sub oo/ mode and 12.4 W in multimode.

High-power efficient continuous-wave TEM 00 intracavity frequency-doubled diode-pumped Nd:YLF laser

We describe experimental results with a diode-pumped, intracavity-doubled cw Nd:YLF laser in multilongitudinal mode and TEM00 spatial transverse mode with a critical phase-matched lithium triborate crystal. Taking into account the thermal effects of Nd:YLF, energy-transfer upconversion, and the thermal fracture limit, we set up a power-scaling model to optimize and design a fundamental diode-pumped Nd:YLF laser. A highly efficient second-harmonic laser was achieved, based on the optimized cavity design. A second-harmonic-generation output power of 20.5 W at a wavelength of 527 nm was obtained at an incident pump power of 60 W, corresponding to an optical-to-optical efficiency of 34.2%. The TEM00 mode green laser operates at a measured M2 parameter of 1.2. The instability of the green laser power is less than +/- 1% RMS.

Hybrid holographic microscope for interferometric measurement of microstructures

A hybrid holographic microscope is proposed to realize both imaging and interferometric measurement of microstructures and meet the two major challenges in micromeasurement, which are high resolution and small object size. The system is developed based on an in-line digital holographic arrangement incorporated with a long-distance microscope. Studies demonstrate that it is capable of realizing measurement of microstructures with lateral dimensions of at least 5 ?m. The application of the system in micromeasurement is experimentally investigated on a microcantilever. The load-induced deflection is obtained and validated with numerically simulated results based on a finite element model. Error analysis for displacement determination is performed.

Highly efficient high-repetition-rate tunable all-solid-state optical parametric oscillator

A compact highly efficient intracavity optical parametric oscillator (OPO) is demonstrated by placing an OPO cavity in a diode-pumped Q-switched Nd:YLF laser. By taking into account the thermal and energy upconversion transfer effects of Nd:YLF, a power scaling model is set up to optimize and design a fundamental mode diode-pumped Nd:YLF laser. The linearly polarized Nd:YLF laser in the 1053-nm line generates 6.5, 9.2, and 11.9 W average power in TEM/sub 00/ mode at the repetition rates of 1, 2, and 4.5 kHz, respectively. A highly efficient OPO signal output is realized by using an optimized cavity design. We obtain OPO average signal powers of more than 1 W over a broadband continuous tuning range from 1650 to 2100 nm at the repetition rate of 1 kHz. The OPO signal pulsewidth is 10 ns at 1 kHz. Signal energies of 2.5, 1.55, and 0.67 mJ per pulse are produced at the repetition rates of 1, 2, and 4.5 kHz, respectively. The maximum OPO signal energy of 2.5 mJ per pulse is obtained at 1900 nm, where the pump energy is 6 mJ per pulse and the pump-to-signal efficiency is 42%. A maximum average signal power of 4.1 W is generated at 3.5 kHz.

Thermal lensing effects for diode-end-pumped Nd:YVO4 and Nd:YAG lasers

The thermal effects of diode-end-pumped Nd:YVO4 and Nd:YAG lasers are analyzed by a 2-D analytical model and a 3-D numerical model, respectively. The characteristics of the Nd:YVO4 and Nd:YAG rods are comparatively discussed, including temperature distributions and thermal lensing effects. The model quantitatively separates the end effect from the index parts. Under the same pump configuration, we find that the temperature increase in the Nd:YVO4 laser was 2 times larger than those in the Nd:YAG laser, while the thermal lensing coefficient of Nd:YAG was 27% higher than that of Nd:YVO4. We also conclude that a good roundness of the Nd:YVO4 laser beam profile is expected because nearly symmetric thermal lensing in the a and c axes is predicted from our modeling and measurement. Correspondingly, we perform experiments that are in good agreement with theoretical modeling for the thermal lensing of diode-pumped Nd:YVO4 and Nd:YAG lasers.

Heating measurements in diode-end-pumped Nd:YVO4 lasers

Thermal effects are investigated using laser interferometric and infrared thermometric techniques in diode-end-pumped solid state lasers operating under lasing and nonlasing states. The laser interferom- etry provides high resolution to determine the optical path difference (OPD) induced by thermal effects of the crystal. The infrared thermom- eter measuring the temperature distribution at the end pump face is pro- posed to evaluate the thermal lensing power induced by the index part. It is found that the temperature variations across the crystal face under nonlasing operation increase 20 to 27% compared with lasing conditions in Nd:YVO4 crystals. Thermal lensing power under the actual lasing con- dition is measured to be 0.2 m 21 to 2.1 m 21 with the pump power ranging from 1 to 8.3 W; the index part takes the proportion of about 59% in the total thermal lensing effect for end-pumped Nd:YVO4 lasers.

Characterization of microstructures with in-line digital micro-holo-interferometry

With respect to the two major challenges in micromeasurement, which are high resolution and small object size, digital micro-holo-interferometry is proposed in this paper to provide quantitative information on load-induced variations of microstructures under testing. As primarily measured properties, the obtained deflection-load relationship enables subsequent accurate determination of strain and stress. More importantly, properties of materials in the micro level, which are known different greatly from those of identical bulky ones, can be evaluated based on these experimental input data for computational simulations. Developed upon the in-line configuration and incorporated with long distance microscope, the proposed system can achieve higher imaging performance and resolution capacity. Studies demonstrate that it is capable of realizing accurate measurement to microstructures with lateral dimensions of at least 8 microns. Its applications in characterization of microstructures are experimentally investigated on a micro cantilevered beam as an example. The load-induced deflection is obtained and validated with numerical simulated results based on finite element analysis.