Amplitude-modulated Laser Light Research Articles

An Optically Pumped Magnetometer with Omnidirectional Magnetic Field Sensitivity.

In mobile applications such as geomagnetic surveying, two major effects hamper the use of optically pumped magnetometers: dead zones, sensor orientations where the sensors signal amplitude drops; and heading errors, a dependence of the measured magnetic field value on the sensor orientation. We present a concept for an omnidirectional magnetometer to overcome both of these effects. The sensor uses two cesium vapor cells, interrogated by circularly-polarized amplitude-modulated laser light split into two beams propagating perpendicular to each other. This configuration is experimentally investigated using a setup wherein the laser beam and magnetic field direction can be freely adjusted relative to each other within a magnetically shielded environment. We demonstrate that a dead-zone-free magnetometer can be realized with nearly isotropic magnetic-field sensitivity. While in the current configuration we observe heading errors emerging from light shifts and shifts due to the nonlinear Zeeman effect, we introduce a straightforward approach to suppress these systematic effects in an advanced sensor realization.

Financial states of world financial and commodities markets around sovereign debt crisis

Measurement of reflected optical light from the surface of microscopic structures can offer a precise tool to investigate the oscillatory motion of micro- and nano-mechanical resonators. We demonstrate an optical investigation of multiple flexural modes of a nanomechanical resonator in thermal motion based on amplitude-modulated laser light, with higher-order resonant modes up to ~ 17 MHz measured with this technique. Scanning focused laser light reproduces the mode shape of each flexural mode, and theoretical estimation of measurement sensitivity shows a displacement sensitivity of ~ 10 −15 m/Hz1/2. For nanomechanical devices incompatible with interferometric systems, this technique can exhibit sufficient sensitivity to explore small motional states.

Magnetometry with millimeter-scale antirelaxation-coated alkali-metal vapor cells

Dynamic nonlinear magneto-optical-rotation signals with frequency- and amplitude-modulated laser light have been observed and investigated with a spherical glass cell of 3 mm diameter containing Cs metal with inner walls coated with paraffin. Intrinsic Zeeman relaxation rates of γ/(2π)≈20 Hz and lower have been observed. Favorable prospects of using millimeter-scale coated cells in portable magnetometers and secondary frequency references are discussed.

A Study on the Accuracy of the Phase Measurement of Distance-Measuring Instrument Using Amplitude-Modulated Laser Light.

We intended to develop a distance-measuring instrument with accuracy of ±0.3 mm atseveral meter range, using an amplitude-modulated diode laser light as a carrier light beam and a heterodyne phase comparating system. With this setup we studied error factors which influence the accuracy of the phase measurement. The study essentially aimes (i) to ensure asufficient signal-to-noise ratio for received signals, (ii) to take care of input levels of mixer circuits, (iii) to ensure a rigorous symmetry about a beam splitter for two optical paths, and (iv) to ensure a symmetry for two radio-frequency signal circuits. Results are satisfactory.

High frequency heterodyne spectroscopy with current-modulated diode lasers

The use of current-modulated semiconductor lasers for optical heterodyne spectroscopy has been investigated at modulation frequencies up to 2.6 GHz. The current modulation produces a simultaneous frequency and amplitude modulation of the laser output. A characteristic heterodyne spectrum occurs when the modulated laser probes a narrow absorption line. In order to analyze the measured spectra, a complete line shape theory has been derived for heterodyne spectroscopy with frequency- and amplitude-modulated laser light. The results show how the obtained signals depend on the modulation frequency and the phase difference between the frequency and the amplitude modulation. The measurement technique permits investigation of the FM-AM phase difference of current-modulated semiconductor lasers.

Waveform of the amplitude modulated laser light by means of a mechanical chopper

The model which reproduces accurately and conveniently the waveform of the amplitude modulated cw laser light in the TEM00 mode by a mechanical chopper is developed. It is found that its predictions are in good agreement with the strict, completely accurate but rather lengthy and inconvenient treatment and with the experiment. The criterion for the efficient amplitude modulation of the laser light by a mechanical chopper is formulated.

Resonance fluorescence spectrum of intense amplitude modulated laser light

The spectrum of light scattered by a two-level atom in an intense laser beam with amplitude modulated light is calculated. The spectrum is obtained by evaluating the Fourier transform of the autocorrelation function of the dipole moment. The modulation introduces new components in both the coherent and incoherent parts of the frequency distribution. The total intensity when plotted as a function of the average Rabi frequency exhibits parametric resonances.

Measurement of the shot-noise reduction factor

A new method of measurement of the shot-noise reduction factor at microwave frequencies is described. In this method, the reduction factor is measured by using a traveling-wave phototube with oxide-coated cathode, on which an amplitude-modulated laser light is projected. The photodetected outputs, with and without the space-charge cloud near the cathode, are compared. The shot-noise reduction factor is calculated from the ratio of both outputs. Measuring frequencies (1320 and 2640 MHz), beam current density, cathode temperature, and beam-forming-electrode voltage are varied in this measurement. The shot-noise reduction factor tends to small values at lower frequency, and goes to unity at higher frequency than the plasma frequency at the potential minimum. The experimental results agree in general with the theoretical calculations done by Whinnery, Tien, and Moshman. The physical meaning of these results is interpreted as a plasma oscillation at the potential minimum. Also, the relation between the "Currie gun" and the shot-noise reduction is discussed.