Lasers Revolutionized Optical Metrology
As in many other fields of optics and photonics, the advent of lasers has brought about a revolutionary change in optical metrology. Since early times, the principles of optical metrology have been based on ingenious use of various physical parameters that characterize light, such as amplitude, phase, optical frequency, wavelength, coherence, and the state of polarization. Therefore the crux of problems in optical metrology lies in how precisely these parameters can be controlled so that object information can be best encoded into and decoded from light with the highest possible precision.
The use of lasers marked the first time in the long history of optical metrology that such highly controlled light became available for use as a physical medium in object information acquisition for sensing and measurement, and it opened up new possibilities in optical metrology. For example, frequency-stabilized lasers, with their light strongly confined in the spectral domain, provided a well-defined standard for dimensional metrology, while ultrashort pulse lasers, with their light strongly confined in the time domain, enabled high-resolution range finding.
The impact of lasers on optical metrology is too large and too widespread to be covered in this short article. We therefore restrict our focus to how lasers have changed industrial interferometry, where the term interferometry is used in the broad sense to include holographic interferometry and speckle interferometry.
The introduction of the laser as a light source for optical interferometry was of particular importance for industrial applications. Heterodyne interferometry, using a shot-noise-limited beat signal between two different optical frequencies of laser light generated by a Bragg cell or a stabilized Zeeman laser, permitted ultrahigh-resolution surface profilometry with height resolution reaching ~1 Angstrom. Laser interferometry, based on homodyne phase detection by the phase-shift technique or the spatial-carrier technique combined with modern fringe analysis, is now widely used for full-field testing of optical surfaces.
The strength of phase-measuring laser interferometry lies in its ability to determine the phase down to the range of nanometers. However, this strength is accompanied by the weakness that the detected phase is wrapped into the range of (-π,π) with ambiguity by integer multiples of 2π. Because of this phase ambiguity, one fails in measurement when an object under test, e.g., a micromachine, has large discontinuities corresponding to phase jumps more than π.
Spectral domain interferometry using an optical-frequency tunable laser has given us a solution to this problem, which arose from the monochromaticity of laser light. Spectral fringes, generated by scanning the optical frequency of the frequency-tunable laser, carry unambiguous depth information about the discontinuous object heights in their fringe frequencies. This technique of spectral interferometry making use of the frequency tunability of lasers formed the basis of today's advanced frequency domain optical coherence tomography (OCT).
Another significant change to optical metrology brought about by the laser was the expansion of optical interferometry to include interference of random optical fields scattered from diffuse surfaces or propagated through a turbid medium.
Of utmost importance in this respect was the emergence of holographic interferometry and speckle metrology in which the coherence of laser light plays a fundamental role. The ability of holography to record and reconstruct the optical field from a 3D object permits interference between two optical fields recorded at different instants of time before and after the object is deformed and/or displaced.
Another important feature that distinguishes holographic interferometry from conventional interferometry is that it is applicable to general objects with unpolished surfaces. These unique features of holographic interferometry have found many successful industrial applications best exemplified by the nondestructive testing of a tire for an automobile. An alternative holographic technique, often used for vibration analysis, is time-average holography. A vibrating surface is recorded in a hologram with a recording time longer than the period of vibration so that only the static interference fringes generated by the light from the vibration nodes are recorded in the hologram while the dynamic fringes generated by the light from the vibration loops are averaged out during the long recording time. Thus the reconstructed image visualizes the vibration modes as a fringe contour map.
The use of a short-pulse laser offers another possibility of light-in-flight recording by holography. A short pulse reference beam illuminating the hologram at an oblique angle serves as a time-gating window as it traverses the hologram so that each part of the hologram reconstructs the object at a different instant of time and enables the visualization of light in flight.
Recent advances in CCD and CMOS technologies have made it possible to record a hologram digitally with a high-resolution image sensor and to reconstruct the optical field numerically with a computer or optically by a spatial light modulator. This technique of digital holography has added such a new function to holographic interferometry that two optical fields recorded at different times and different locations can be compared directly on the basis of numerical data by performing arbitrary numerical operations such as propagation and focusing of light and aberration corrections.
Besides holographic interferometry, holography has introduced another important function of wavefront shaping into optical interferometry. With a computer-generated hologram, one can synthesize a desired aspheric wavefront which serves as a prototype standard for the interferometric testing of an aspheric surface.
Speckle phenomena have been known for a long time, ever since lasers came to be used for holography and optical information processing. These laser-induced granular intensity distributions, resulting from the coherent superposition of many random optical fields scattered from diffuse surfaces, were first regarded as noise or a nuisance that unavoidably appears when imaging is performed with laser light. Soon researchers became aware that random laser speckles carry useful information about the object surface by which laser light is scattered.
In speckle photography, speckle patterns observed on the object's surface are used as unique markers that indicate individual locations on the object surface, and deformation and/or displacement of the object are detected from the local movement of these speckle markers.
Speckle interferometry makes use of the interference between the random optical field on a diffuse object surface created by scattering and the reference optical field, which can have either a smooth or random wavefront. Unlike conventional interferometry, which gives the object information directly with a fringe contour map, the random speckle pattern observed on the object surface does not give any direct information about the object by itself. However, when the object is deformed, e.g., by thermal loading or by external force, the speckle pattern changes its distribution due to the change in the optical path difference introduced by the deformation. The difference between the two speckle patterns before and after the deformation reveals the distribution of the deformation, which is usually visualized as a fringe contour map by rectifying the 2D difference distribution.
With the combined use of high-resolution image sensors and modern fringe analysis, speckle metrology has now become an indispensable means of nondestructive testing for industrial applications and civil engineering.
Even with these very limited instances described above, we can see the large impact the laser has had in the evolution of modern optical metrology for industrial applications. Together with our colleagues in the field of industrial optical metrology, we would like to celebrate the 50th anniversary of laser.
Germany's Theodor Hänsch shared half the 2005 Nobel Prize in Physics with American John Hall for development of laser-based precision spectroscopy, including the optical frequency comb technique.
The frequency comb technique made it possible to measure with extreme precision the number of light oscillations per second, with an accuracy of fifteen digits.
The laser-based measurement technology has deepened our knowledge of the properties of matter, space, and time and brought advancements to extremely accurate optical clocks and satellite-based navigation systems such as GPS.
In additional to numerous applications in interferometry, microscopy, spectroscopy, etc. lasers are used to:
- Measure gas and fluid flow in automobiles
- Sense acceleration and rotation
- Study airflow around aircraft, missiles, and projectiles
- Ensure smooth surfaces on camera lenses
- Measure distance and displacement
- Measure temperature and pressure variation
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