Picometer Resolution Research Articles

Frequency stabilization of a multimode high-power He-Ne laser

A high-power, frequency-stabilized laser source is required for an atomic force microscope which uses interferometric techniques to obtain distance measurements with picometer resolution. For this purpose, the oscillation frequency of a 10 mW multimode He-Ne laser was stabilized using a new technique. Optical interaction between the longitudinal modes synthesizes intermode beats. Frequency of the intermode beat signal changes periodically with respect to cavity thermal expansion. This phenomenon is explained by the concept of ‘‘frequency pulling.’’ The secondary beats synthesized by the interaction between the intermode beat signals also change due to frequency pulling. The relation between the secondary beat of an intermode beat frequency and the laser cavity length is utilized for the stabilization of laser frequency. The change in the frequency of the secondary intermode beat is employed as the feedback signal to control the length of the cavity of the laser. To detect the frequency change in the secondary beat signal, a simple microwave electronics circuit was designed. An excellent frequency stability (instability: ±2 parts in 108) and high-power laser output were obtained successfully using this simple technique.

Servopositioning with picometer resolution

A macroscopic one-axis movement with picometer resolution was made possible by a feedback loop driving a piezoelectric element locking an elastic guide to a reference value, or to a reference path. The guide displacement has been measured by a high-resolution optical interferometer. Not only were prompt and accurate positionings obtained, but disturbances due to seismic noise were also compensated for. In contrast to conventional positioning, driver linearity and stability, antivibration support, and maximum displacement are not subjected to strict conditions. The performances of an x-ray interferometer were greatly improved by this feedback: the positioning with 1-pm resolution over 30-Hz bandwidth and 0.1-mm maximum displacement was made possible.

Phase Modulation in High-resolution Optical Interferometry

The measurement of nanometre displacements with picometre resolution has been made possible by a phase-modulation recovery scheme applied to an optical Michelson interferometer using polarization encoding. In contrast to conventional schemes, phase modulation is carried out before the interferometer optics. In this way, the low-frequency components of the phase shift between the two interfering beams are locked at zero (within limits set only by shot noise) before beam splitting by a feedback loop driving the modulator. An interferometer prototype, illuminated by a beam thus modulated, was constructed and coupled to an X-ray interferometer to compare the optical and X-ray interferometric measurement values of sub-nanometre displacements. A resolution better than 1 pm over a 100 Hz bandwidth was obtained.

Preliminary results with a high-resolution inductively coupled plasma Fourier transform spectrometer

This paper presents some preliminary results obtained by monitoring the optical emission from an argon radiofrequency inductively coupled plasma using a Fourier transform spectrometer. The system is shown to be capable of obtaining high (picometer) resolution data over a large wavelength range (>100 nm) in a relatively short period of time (ca. 6 min). The possibility of monitoring several wavelengths simultaneously in order to provide an averaged emission value for a particular element is analysed and results are presented to show that this can offer some improvement in signal to noise ratio.