The present state of high-resolution displacement measuring interferometry is reviewed. Factors which determine the accuracy, linearity and repeatability of nanometre-scale measurements are emphasized. Many aspects of interferometry are discussed, including general metrology and alignment errors, as well as path length errors. Optical mixing and the nonlinear relation between phase and displacement are considered, as well as the influence of diffraction on accuracy. Environmental stability is a major factor in the repeatability and accuracy of measurement. It is difficult to obtain a measurement accuracy of 10-7 when working in air. Several approaches to improving this situation are described, including multiwavelength interferometry. Recent measurements of the short- and long-term frequency stability of lasers are summarized. Optical feedback is a subtle, but important source of frequency destabilization, and methods of detection and isolation are reviewed. Calibration of phase measuring electronics used for subfringe interpolation is included. Progress in 'in situ' identification of error sources and methods of validating accuracy are emphasized.

http://iopscience.iop.org/article/10.1088/0957-0233/4/9/001/pdf

Recent advances in displacement measuring interferometry

The field backscattered from a remote surface into the laser cavity induces an efficient modulation, both in amplitude and frequency, of the cavity field. The modulating signals are the interferometric components (sin and cos) of object optical path length, which can be recovered by heterodyne detection through a separate reference‐frequency mode. Using a dual polarization Zeeman stabilized He‐Ne laser, a compact self‐aligning interferometer is developed.

Laser interferometry by induced modulation of cavity field

Tuning characteristics of power, intramode beat frequency, and polarization of 633-nm internal mirror lasers filled with natural Ne or 20Ne are studied. It is found that the tuning curve of a polarization signal is S-shaped for the natural Ne laser, whereas that of the intramode beat frequency is S-shaped for the 20Ne laser. The characteristic parameters of the tuning curve of the intramode beat frequency are determined. The axial modes of a 3He–20Ne laser are collapsed into one by a transverse magnetic field and frequency stabilized by cooling the laser tube with a fan to keep the intramode beat constant. Frequency fluctuation of 3 × 10−10 is obtained by the Allan variance analysis for the integration period range of 0.1–100 sec. Absolute accuracy and interinstrumental reproducibility of stabilized wavelengths can be expected for a 3He–20Ne transverse Zeeman laser stabilized at the central frequency of the intramode beat.

Stabilized (3)He-(20)Ne transverse Zeeman laser.

Discusses the practical design and performance of lasers stabilised by balancing the powers in two orthogonally polarised modes. Procedures for aligning the optical system and for optimising the electronic controller are described. The performances of several types of commercial tubes have been tested over periods of over three years. A short-term (1 s) stability of 20 kHz and a long-term (several months) stability of 10 MHz have been achieved with some tubes. An additional characteristic of these lasers is their good power stability (within 0.1% over several hours).

https://iopscience.iop.org/article/10.1088/0022-3735/16/12/024/pdf

Two-mode frequency-stabilised He-Ne (633 nm) lasers: studies of short- and long-term stability

An electro-optical apparatus transforms a single frequency, linearly polarized laser input beam (18) from a light source (10) into an output beam (36) having two collinear orthogonally polarized output beam components (32,33) differing in frequency from each other by the frequency of a stabilized electrical signal (40) provided from an electronic oscillator (38) The output of the oscillator (38) is provided to a power amplifier (42) which is used to drive a piezoelectric transducer (46) affixed to an acousto-optical Bragg cell (52) through which the input beam (18) passes and is transformed into the output beam (36) composed of two beam components (32,33) The electrical output (44) of the power amplifier (42) is adjusted so that each of the output beam components (32,33) has approximately a preselected fraction of the intensity of the input beam (18)

Apparatus to transform with high efficiency a single frequency, linearly polarized laser beam into beams with two orthogonally polarized frequency components orthogonally polarized

Internal-mirror 6328-A He-Ne lasers have an intramode frequency difference when placed in a transverse magnetic field. When the magnetic field strength is increased until the Zeeman frequency shift approaches the mode spacing, mode coupling is nduced. In some multimode lasers, this mode coupling results in a collapse into single-mode operation with no loss of over-all laser power in the output. The intramode frequency difference can be exploited to produce a simple frequency stabilization system whose frequency variation is less than 1 part in 10(10). Plots of beat frequency variation and single-mode tuning range are included.

R. H. Morris

Papers

Single-mode collapse in 6328-A He--Ne lasers

Frequency stabilization of internal mirror He-Ne lasers in a transverse magnetic field.