High Precision Detection of Change in Intermediate Range Order of Amorphous Zirconia-Doped Tantala Thin Films Due to Annealing.

Understanding the local atomic order in amorphous thin film coatings and how it relates to macroscopic performance factors, such as mechanical loss, provides an important path towards enabling the accelerated discovery and development of improved coatings. High precision x-ray scattering measurements of thin films of amorphous zirconia-doped tantala (ZrO_{2}-Ta_{2}O_{5}) show systematic changes in intermediate range order (IRO) as a function of postdeposition heat treatment (annealing). Atomic modeling captures and explains these changes, and shows that the material has building blocks of metal-centered polyhedra and the effect of annealing is to alter the connections between the polyhedra. The observed changes in IRO are associated with a shift in the ratio of corner-sharing to edge-sharing polyhedra. These changes correlate with changes in mechanical loss upon annealing, and suggest that the mechanical loss can be reduced by developing a material with a designed ratio of corner-sharing to edge-sharing polyhedra.

An instrument to measure mechanical up-conversion phenomena in metals in the elastic regime.

Crystalline materials, such as metals, are known to exhibit deviation from a simple linear relation between strain and stress when the latter exceeds the yield stress. In addition, it has been shown that metals respond to varying external stress in a discontinuous way in this regime, exhibiting discrete releases of energy. This crackling noise has been extensively studied both experimentally and theoretically when the metals are operating in the plastic regime. In our study, we focus on the behavior of metals in the elastic regime, where the stresses are well below the yield stress. We describe an instrument that aims to characterize non-linear mechanical noise in metals when stressed in the elastic regime. In macroscopic systems, this phenomenon is expected to manifest as a non-stationary noise modulated by external disturbances applied to the material, a form of mechanical up-conversion of noise. The main motivation for this work is for the case of maraging steel components (cantilevers and wires) in the suspension systems of terrestrial gravitational wave detectors. Such instruments are planned to reach very ambitious displacement sensitivities, and therefore mechanical noise in the cantilevers could prove to be a limiting factor for the detectors' final sensitivities, mainly due to non-linear up-conversion of low frequency residual seismic motion to the frequencies of interest for the gravitational wave observations. We describe here the experimental setup, with a target sensitivity of 10(-15) m/Hz in the frequency range of 10-1000 Hz, a simple phenomenological model of the non-linear mechanical noise, and the analysis method that is inspired by this model.

Evaluating the effect of transmissive optic thermal lensing on laser beam quality with a shack-hartmann wave-front sensor.

We examine wave-front distortion caused by high-power lasers on transmissive optics using a Shack-Hartmann wave-front sensor. The coupling coefficient for a thermally aberrated Gaussian beam to the TEM(00) mode of a cavity was determined as a function of magnitude of the thermally induced aberration. One wave of thermally induced phase aberration between the Gaussian intensity peak and the 1/e(2) radius of the intensity profile reduces the power-coupling coefficient to the TEM(00) mode of the cavity to 4.5% with no compensation. With optimal focus compensation the power coupling is increased to 79%. The theoretical shape of the thermally induced optical phase aberration is compared with measurements made in a neutral-density filter glass, Faraday glass, and lithium niobate. The agreement between the theoretical and the measured thermal aberration profiles is within the rms wave-front measurement sensitivity of the Shack-Hartmann wave-front sensor, which is a few nanometers.

Focal plane position detection with a diffractive optic for shack-hartmann wave-front sensor fabrication.

During construction of a Shack-Hartmann wave-front sensor it is critical that the spacing between the lens array and the detector array be equal to the lens array focal length to obtain accurate and precise measurements of a wave front. This separation is often difficult to determine with large f/# lenses, because their focal spot diameter does not change substantially for small displacements on either side of the focal plane. We describe a method with an array of off-axis lens segments for determining the location of the focal plane. Because the lenses are off axis, changes in the distance from the optic to the detector array result in transverse focal spot position variations as a function of their separation from the lenses. By analyzing the focal spot pattern on a CCD, we achieved 12-mum rms error in the axial position measurement while moving a 4-mm-focal-length optic over 1 mm.

Spatial and temporal filtering of a 10-W Nd:YAG laser with a Fabry--Perot ring-cavity premode cleaner.

We report on the use of a fixed-spacer Fabry-Perot ring cavity to filter spatially and temporally a 10-W laser-diode-pumped Nd:YAG master-oscillator power amplifier. The spatial filtering leads to a 7.6-W TEM (00) beam with 0.1% higher-order transverse mode content. The temporal filtering reduces the relative power fluctuations at 10 MHz to 2.8x10(-9)/ radicalHz, which is 1 dB above the shot-noise limit for 50 mA of detected photocurrent.

Quantum noise in a continuous-wave laser-diode-pumped Nd:YAG linear optical amplifier.

We present measurements of the power noise that is due to optical amplification in a laser-diode-pumped Nd:YAG free-space traveling-wave linear amplifier in a master-oscillator-power-amplifier configuration. The quantum noise behavior of the optical amplifier was demonstrated by use of InGaAs photodetectors in a balanced detection configuration, at a total photocurrent of 100 mA and in a frequency band from 6.25 to 15.625 MHz. The experimental results are in good agreement with predictions.

Polarization-based balanced heterodyne detection method in a Sagnac interferometer for precision phase measurement.

We describe a balanced heterodyne detection method for a Sagnac interferometer that uses a polarization-dependent beam splitter. The signal and the local oscillator are orthogonally polarized components of a single laser beam, permitting the detection of the signal by subtraction of two photocurrents produced in appropriate polarization projections. Using this scheme, we experimentally demonstrate a phase measurement with a sensitivity of 9x10(-10) rad/ radicalHz. The measurement is robust in the presence of laser frequency noise, as a result of preserving the common-path nature of the Sagnac interferometer, and of laser-amplitude noise, as a result of balanced detection.

Balanced heterodyne signal extraction in a postmodulated Sagnac interferometer at low frequency.

We describe a balanced-heterodyne postmodulated Sagnac interferometer signal extraction method that is suitable for gravitational wave detection. The method is simple to implement by placement of a polarization-selective modulator after the beam splitter in the dark port of the interferometer. The postmodulated Sagnac interferometer retains its common path advantage and exhibits insensitivity to laser frequency noise below, at, and above the heterodyne frequency. Balanced detection reduces sensitivity to laser amplitude noise. In this scheme mirror displacement signals were rf demodulated and observed from 0.2 to 10kHz.

Long-term stability of two diode-laser-pumped nonplanar ring lasers independently stabilized to two Fabry-Perot interferometers.

We have locked two 1064-nm diode-laser-pumped Nd:YAG lasers to two ultralow-expansion glass-ceramic Fabry-Perot interferometers by using the Pound-Drever discriminator technique. The interferometers have finesses of approximately 200,000 and 5-kHz linewidths and are housed in separate temperature-stabilized vacuum vessels. Allan variance measurements of the beat note between the two lasers are as low as 10(-14) for delay times between 0.5 and 2 s and increase with the time interval for times longer than 2 s, an improvement of 4 orders of magnitude over the free-running performance. Daily variations of the beat-note frequency are less than 1 MHz, which corresponds to a relative temperature change of approximately 100 mK between the interferometers.

Absolute frequency stabilization of diode-laser-pumped Nd:YAG lasers to hyperfine transitions in molecular iodine.

Diode-laser-pumped Nd:YAG lasers were frequency stabilized by locking their frequency-doubled output to Doppler-free absorption lines of the (127)I(2) molecule. The successive two-sample deviation of the beat frequency between two independent iodine-stabilized lasers is less than 650 Hz, or 2.3 x 10(-12) of the laser frequency, for averaging times between 24 and 80 s.

Optical biosensor assay (OBA).

We describe a new biosensor immunoassay involving optical diffraction to detect clinically important analytes in human body fluids. A silicon wafer is used as a support for immobilization of antigen or antibody. The protein-coated surface is illuminated through a photo mask to create distinct periodic areas of active and inactive protein. When the surface is incubated with a positive sample, antigen-antibody binding occurs only on the active areas. Upon illumination with a light source such as a laser, the resulting biological diffraction grating diffracts the light. A negative sample does not result in diffraction because no antigen-antibody binding occurs to create the diffraction grating. The presence or absence of a diffraction signal differentiates between positive and negative samples, and the intensity of the signal provides a quantitative measure of the analyte concentration. The technique is demonstrated with a quantitative assay of choriogonadotropin in serum.

6.5-W, 532-nm radiation by cw resonant external-cavity second-harmonic generation of an 18-W Nd:YAG laser in LiB(3)O(5).

We report 6.5 W of TEM(00) cw 532-nm radiation generated by external resonant doubling in a LiB(3)O(5) crystal. An 18-W, cw, injection-locked single-frequency Nd:YAG laser was used to generate 532-nm second-harmonic output with a 3-dB heterodyne linewidth of 15 kHz.

Active frequency stabilization of a 1.062-microm, Nd:GGG, diode-laser-pumped nonplanar ring oscillator to less than 3 Hz of relative linewidth.

We present results on the frequency stabilization of two diode-laser-pumped ring lasers that are independently locked to the same high-finesse interferometer. We measure the relative frequency stability by locking the lasers one free spectral range apart and observing the heterodyne beat note. The resultant beat note width of 2.9 Hz is consistent with the theoretical system noise-limited linewidth and is approximately 20 times that expected for shot-noise-limited performance.

Injection locking of a 13-W cw Nd:YAG ring laser.

A lamp-pumped, 13-W cw Nd:YAG ring laser at 1.064 microm is injection locked using a 40-mW single-frequency diodelaser-pumped Nd:YAG laser as the master oscillator. The phase fidelity of the injected slave to the master is measured using an all-optical technique.

Efficient monolithic MgO:LiNbO(3) singly resonant optical parametric oscillator.

We have operated a monolithic MgO:LiNbO(3) singly resonant optical parametric oscillator (OPO) as both a standing-wave and a ring-geometry resonator. The OPO was pumped by the second harmonic of an amplified single-mode diode-laser-pumped Nd:YAG laser. Pump depletions of greater than 60% were observed when pumping four times greater than the 35-W threshold. The OPO output at the resonant signal tuned with temperature from 834 to 958 nm, while the corresponding idler tuned from 1.47 to 1.2 microm. The spectral characteristics of the OPO signal output and the relative merits of a standing wave versus a ring geometry are discussed.

Coherent anti-Stokes Raman scattering from small volumes.

We have used coherent anti-Stokes Raman scattering spectroscopy to resolve the Raman Q-branch line [Q (2) at 2987.18 cm(-1)] of dueterium gas contained in a 60-microm-diameter glass sphere. The 60-microm spheres contained only 10(+13) molecules at standard temperature and pressure; this made it possible to study gases that in larger quantities would be too dangerous or expensive to use.

High-resolution continuous-wave coherent anti-Stokes Raman spectroscopy in a supersonic jet.

We have obtained high-resolution cw coherent anti-Stokes Raman spectra of the nu(1) Q branch of methane in an underexpanded supersonic jet at temperatures as low as 31.5 K and pressures below 2 Torr.