David H. McIntyre

Second-harmonic generation and optical stabilization of a diode laser in an external ring resonator

The second harmonic of the 842-nm output of a GaAlAs diode laser is generated in a KNbO(3) crystal in a resonant, external ring cavity. The diode laser is optically stabilized to the ring cavity through feedback from the counterpropagating fundamental wave, which is weakly excited in the resonator. We have produced 6.7 mW of tunable, narrowband radiation at 421 nm and have used that light to perform saturation spectroscopy on narrow transitions in rubidium.

Optically stabilized narrow linewidth semiconductor laser for high resolution spectroscopy

Abstract A narrow bandwidth tunable semiconductor laser system operating near 780 nm is described. The commercial laser diodes are frequency stabilized by optical feedback from an external, confocal Fabry-Perot resonator. The feedback phase is electronically stabilized to improve the frequency stability. A beat signal with 30 kHz linewidth between two identical, independent systems is recorded. The performance of this system is demonstrated in a laser cooling experiment with rubidium.

Stabilized diode-laser system with grating feedback anf frequency-offset locking

Abstract Reported is a diode-laser system that utilizes optical feedback from a diffraction grating to stabilize the diode-laser frequency and to reduce the linewidth. Electronic control is also incorporated to compensate for mechanical and thermal noise. The resultant system can be tuned ± 5 nm from the free-running laser wavelength, can be continuously scanned up to 10 GHz, and has a linewidth of approximately 150 kHz as determined by observing the heterodyne beat note between two systems. Results are also presented of frequency-offset locking of two lasers using a simple electronic frequency discriminator that relies on a digital delay generator. The frequency difference between two lasers can be continuously tuned from 2 to 30 MHz.

Diode-laser noise spectroscopy of rubidium

We report on spectra obtained by measuring the laser intensity noise after a broad-bandwidth diode-laser beam passes through a rubidium vapor cell. The atomic resonance converts laser frequency fluctuations into intensity fluctuations. We compare our experimental spectra with numerically calculated spectra based on a phase-diffusion model of the laser field and find good agreement.

Micromechanics of cellularized biopolymer networks.

Significance Mechanical interactions between cells, mediated by the elastic response of the extracellular matrix to active applied forces, play a critical role in developmental biology, wound healing, and cancer progression. This work applies sophisticated technical means, both in experiment and computational modeling, to investigate the micron-scale mechanics of a popular model of this medium, a collagen gel. The results obtained show clearly that on the cellular scale, there are significant spatial variations in the micromechanics due to network heterogeneities. Collagen gels are widely used in experiments on cell mechanics because they mimic the extracellular matrix in physiological conditions. Collagen gels are often characterized by their bulk rheology; however, variations in the collagen fiber microstructure and cell adhesion forces cause the mechanical properties to be inhomogeneous at the cellular scale. We study the mechanics of type I collagen on the scale of tens to hundreds of microns by using holographic optical tweezers to apply pN forces to microparticles embedded in the collagen fiber network. We find that in response to optical forces, particle displacements are inhomogeneous, anisotropic, and asymmetric. Gels prepared at 21 °C and 37 °C show qualitative difference in their micromechanical characteristics. We also demonstrate that contracting cells remodel the micromechanics of their surrounding extracellular matrix in a strain- and distance-dependent manner. To further understand the micromechanics of cellularized extracellular matrix, we have constructed a computational model which reproduces the main experiment findings.

A blue dye laser with sub-kilohertz stability

Abstract We have stabilized the frequency of a commercial ring dye laser operating in the blue near 486 nm with an internal electro-optic modulator, using optical heterodyne detection of phase-modulated light reflected from a reference cavity. With a servo bandwidth of 10 MHz, the short-term stability has been reduced below 1 kHz. Frequency doubling of this laser will provide highly monochromatic 243 nm radiation for high resolution spectroscopy of the two-photon hydrogen 1S–2S transition.

Using great circles to understand motion on a rotating sphere

Motion observed in a rotating frame of reference is generally explained by invoking inertial forces. While this approach simplifies some problems, there is often little physical insight into the motion, in particular into the effects of the Coriolis force. To aid in the understanding of three-dimensional inertial forces, motion on a rotating sphere is considered from the points of view of an inertial observer and of an observer fixed on the sphere. The inertial observer observes the motion to be along a great circle fixed in the inertial frame, in analogy with simple straight-line motion in the two-dimensional case. This simple “straight-line” viewpoint of the inertial observer is reconciled qualitatively and quantitatively with the view of the rotating observer that requires inertial forces in order to account for the motion. Through a succession of simple examples, the Coriolis and centrifugal effects are isolated and illustrated, as well as effects due to the curvilinear nature of motion on a sphere.

Precision Measurements of the Rydberg Constant

Five recent measurements of the frequencies of optical spectral lines in atomic hydrogen and deuterium have improved the accuracy of the Rydberg constant beyond 1 part in 109. We review these experiments and discuss the importance of the Rydberg constant in atomic theory and in relation to the other fundamental physical constants. We discuss the limitations of these recent experiments as well as the future of Rydberg constant measurements.

Optically stabilized diode laser using high-contrast saturated absorption

Doppler‐free saturation spectroscopy in an optically thick atomic vapor is used in an optical‐feedback stabilization system for a semiconductor diode laser. A portion of the 780 nm diode laser beam passes through a heated rubidium cell and is reflected back to the laser. The optical feedback causes the laser frequency to be stabilized to a hyperfine transition within the Rb D2 line. The linewidth of the laser is reduced by more than two orders of magnitude.

General algorithm to optimize the diffraction efficiency of a phase-type spatial light modulator

We present a general approach for optimizing the diffraction efficiency of a phase-type spatial light modulator (SLM). While the SLM displays a one-dimensional phase grating, the phase shift of one pixel in the grating is varied and the first-order diffraction efficiency is measured. This is repeated pixel-by-pixel to find the optimum phase encoding for the device that maximizes the diffraction efficiency. This method compensates for nonlinearity of the modulator phase response and is especially useful for optimizing modulators with less than 2π phase shift.