Vladimir V. Protopopov

Heterodyne double-channel polarimeter for mapping birefringence and thickness of flat glass panels

A new cross-polarized heterodyne optical technique is developed for two-dimensional (2D) simultaneous mapping of both birefringence and thickness variations in large flat glass panels commonly used in liquid-crystal displays (LCDs). Weak depolarization of a linearly polarized probe beam due to glass birefringence is detected by means of heterodyne mixing of the two cross-polarized and frequency shifted waves generated by Zeeman-type laser. Amplitude variations of the transmitted laser beam due to interference of the partial waves reflected from the both sides of a sample provide information about glass thickness. Measurements are being performed at the intermediate frequency of 2.3MHz, providing several orders of magnitude higher speed of data acquisition with respect to traditional polarimeters. That high speed of measurements makes it possible to perform quality assessment of LCD glass panels not only in few randomly chosen points as it was in common practice before but to obtain entire 2D maps of both ...

Differential heterodyne interferometer for measuring thickness of glass panels

Differential heterodyne interferometer is applied for measuring spatial thickness variations across glass panels of liquid-crystal displays. This system uses the Zeeman laser as a source of two-frequency shifted orthogonally linearly polarized probe waves, passing through the glass in two spatially separated points. These waves are then recombined in a single beam to produce the intermediate frequency signal with the phase proportional to the thickness gradient of a glass sample. The phase of the intermediate signal is measured against the laser reference by means of a lock-in amplifier, and finally real-time integration provides the thickness variation. Since spatial separation of the probe beams is only 1.35 mm good approximation for the thickness gradient is achieved. Detailed design of the interferometer and experimental results on real samples are presented.

Scanning heterodyne Kerr-effect microscope for imaging of magnetic tracks

Design and performance of a new type of Kerr microscope based on heterodyne cross-polarized technique is presented. Weak depolarization of the probe beam due to longitudinal magneto-optical Kerr effect is detected by means of heterodyne mixing of the two cross-polarized and frequency shifted waves generated by Zeeman-type He–Ne laser. In comparison with the traditional homodyne method the proposed technique has better sensitivity and spatial resolution. Experimental results of imaging service magnetic tracks on real samples of magnetic disks are presented, showing better contrast and spatial resolution with respect to the images obtained from commercial devices available in the market.

Laser Heterodyne Radars and Lidars

The title of this chapter is so contradictory that it needs special clarification before commencing the detailed discussion of the chapter matter. The term RADAR emerged in 1941 as an acronym for radio detection and ranging. Thus, the two words “laser radar” being put together represent first contradiction. It sounds even more curios, considering the fact that both words are acronyms: LASER stands for light amplification by stimulated emission of radiation. The combination “laser radar” was introduced in the early 1960s in order to represent new remote-sensing devices based on lasers. The very first laser remote-sensing systems were basically a compilation of traditional radar concepts designed for the same purpose: to detect and measure coordinates of moving remote targets. Much smaller wavelength and shorter pulses ignited enthusiasm of obtaining much more precise measurements with more compact systems. Those first laser systems were designed by people proficient in radar technology, which explains partially why the term “laser radars” was readily accepted. Soon, however, new generation of people came to the scene, educated in the field of optics, and eager to establish their own language in rapidly expanding area of laser systems. Their concern was mainly the peculiar optical phenomena related to optical scattering in atmosphere. Since laser systems developed for this particular purpose had nothing to do with detection and ranging of moving targets, the quest for new term for that kind of laser remote-sensing systems was initiated. As a result, the term LIDAR – light detection and ranging – emerged. It is also possible to encounter the third acronym: LADAR for laser detection and ranging. After some short period of friendly battles between the groups of supporters of these different names for basically very similar technical concepts, the areas of responsibilities were divided as follows: systems designed for tracking of moving targets, space-ships docking, or military applications are now commonly recognized as laser radars, whereas the systems for remote sensing of atmosphere and monitoring wind vectors are referred to as lidars.

Principles of Optical Heterodyning

The relation (1.12) defines the unique configurations of the two interfering waves. If one wave can be considered having some fixed configuration, that is, the reference wave, then the matching condition cannot simultaneously hold true for two or more other waves, coming to the detector at different angles. Therefore, if it is necessary to receive the incoming waves in some finite cone of angles (finite angular field of view), then the amplitude of the heterodyne signal averaged over all possible directions within the field of view is always smaller than the maximum one, corresponding to the matching condition. This decrease in efficiency can be formally ascribed to the decrease in the area of the input optical aperture through which radiation comes to the photo-detector. In other words, it is possible to introduce the so-called effective input aperture. Then a very simple and straightforward relation can be established between the field of view and the effective aperture of the heterodyne receiver. This relation is known as the Siegman antenna theorem [2]. Consider it in more details.

Laser Heterodyne Radiometers

Radiometers are the devices for registration of temporally incoherent radiation, for example, of thermal origin. With the help of radiometers, it is possible to measure temperature of remote objects and to analyze their spectra. In optical domain, there are basically two types of radiometers: with the direct detection receivers and with the heterodyne receivers. The last ones have the advantage of better sensitivity and spectral resolution. Besides, heterodyne radiometers make it possible to create stellar interferometers for precise measurement of angular dimensions of space objects. Characteristics of laser heterodyne radiometers are basically determined by two features: sensitivity of photo-detector and signal processing organization. Insofar as heterodyne de- tectors were analyzed in every detail in Chap. 3, the current chapter is focused on signal processing principles. Specific optical problems are discussed only in the end of the chapter, in conjunction with laser heterodyne interferometers.

Laser Heterodyne Interferometry and Polarimetry

In August of 1970, the Hewlett-Packard technical journal announced a new product: a laser heterodyne interferometer system based on Zeeman two-frequency laser [1]. Through past three decades, the system concept became de facto a standard for precise distance measurement systems, and the laser itself has found numerous applications not only in industry but in scientific research as well. Today, Zeeman lasers are available from two manufacturers: Agilent (the Hewlett-Packard lasers) and Wavetronics [2]. Availability of an inexpensive stable and versatile tool such as the Zeeman two-frequency cross-polarized laser inspired invention of new research areas where heterodyne technology provided new solutions to previous problems. Therefore, prior to addressing the subject of the chapter itself, it is necessary to explain the Zeeman laser functionality. The scheme of the laser is outlined in Fig. 5.1.

Laser Heterodyne Spectroscopy

In 1947, Gorelik [1] and practically at the same time Forrester, Parkins, and Gerjuoy [2] expressed the idea of observing low-frequency interference oscillations (beatings) between two incoherent light sources of slightly different optical frequencies. At that time, any such experiment seemed to be unreal because spectral intensity of traditional light sources was too small. However, 8 years later Forrester, Gudmundsen, and Johnson [3] reported on observation of beatings between σ components of the Zeeman splitting of the 202Hg green line at the wavelength 0.5461 ?m. To explain the difficulties and complexity of the experiment, it would be enough to say that the signal-to-noise ratio was only 10−4 at the frequency of 10 GHz. For detecting the signal, the special cumbersome ultrahigh frequency vacuum photodiode was designed and manufactured. The whole experimental arrangement was so complicated that every practical implementation of this method seemed to be unreal. Nonetheless, even at that time it was clear that, using the light sources of much narrower spectral line and high enough spectral flux, it was possible to significantly simplify the experiment.

Heterodyne Imaging and Beam Steering

Heterodyne receiver coherently transfers the phase of optical wave into the phase of radio signal. This feature is essential for holography and adaptive optics – the fields where phase information is of primary importance. In addition, high sensitivity of the heterodyne receiver together with high angular resolution of optical systems determine such an important application as scanning laser radars for remote imaging. The variety of applications of heterodyne technique to imaging is based on two basic concepts: heterodyne phase-sensitive scanning and heterodyne adaptive enhancement of image quality. Since the final purpose of any imaging technique is high-resolution image, the aforementioned concepts are analyzed in this chapter on the basis of the theory of linear optical systems. Also, phase information delivered by heterodyne receiver makes it possible to create phase-controllable laser arrays for adaptive focusing and steering laser beams. All these issues are discussed in the present chapter.