Lasers spatial coherence

Historically, this has been the most constrained parameter, particularly for confocal laser scanning microscopes that require spatially coherent sources and so have been typically limited to a few discrete excitation wavelengths, traditionally obtained from gas lasers. Convenient tunable continuous wave (c.w.) excitation for wide-held microscopy was widely available from filtered lamp sources but, for time domain FLIM, the only ultrafast light sources covering the visible spectrum were c.w. mode-locked dye lasers before the advent of ultrafast Ti Sapphire lasers. 

A laser is spatially coherent as is a conventional source that is infinitely small. Referring to Figure 8, this may be achieved by moving the observation point P to infinity, at which point a, the angle subtended at P, approaches zero as does the area of emission. We should point out, however, that brightness for a finite source is defined as power per unit area per unit solid angle. Therefore, achieving coherency in this manner reduces the intensity to zero and would require infinite exposure time. Fortunately we do not need perfect coherency, a point that will be treated in more detail later. 

The spatial coherence of the induced emission which renders it possible to focus the laser output into a nearly parallel light beam. 

Owing to aberrations, grating defects, and so on, it may not be adequate to approximate the response function by formulas based on idealized models. If a line source could be found having the spectrum that approximates a 8 function, then perhaps the measurement of such a line would adequately determine the response function. We have learned, however, that the spatial coherence of the source plays an important part in the shape of the response function. This precludes the use of a laser line source to measure the response function applicable to absorption spectroscopy. Furthermore, we... 

The spatial coherence that permits to draw high-resolution images by means of sharply focused laser beams. 

Coherence Coherence is the property of light emitted from a laser such that it is remarkably uniform in color, polarization, and spatial direction. Spatial coherence allows a laser beam to maintain brightness and narrow width over a great distance. 

Moreover, in recent years broad band lasers have appeared which lack any frequency modal structure, at the same time retaining such common properties of lasers as directivity and spatial coherence of the light beam at sufficiently high spectral power density. The advantages of such a laser consist of fairly well defined statistical properties and a low noise level. In particular, the authors of [245] report on a tunable modeless direct current laser with a generation contour width of 12 GHz, and with a spectral power density of 50 /xW/MHz. The constructive interference which produces mode structure in a Fabry-Perot-type resonator is eliminated by phase shift, introduced by an acoustic modulator inserted into the resonator. 

So far in this discussion of diffraction, we have assumed that the periodic object is illuminated by coherent light, such as that produced by a small laser of the type used in the Porter experiments. However, the light produced by a thermal source (e.g., a sodium vapor lamp or a heated filament coupled with a narrow bandpass filter) is never strictly monochromatic even the sharpest spectral line has a finite width. Moreover, such a source has finite extent, and the light is emitted by many independent radiators (atoms). These two characteristics of thermal sources are directly related to what are usually referred to as temporal and spatial coherence, respectively. 

An important characteristic of excimer lasers that sets them apart from traditional UV lasers is their lack of spatial coherence. The interference phenomena that result from the high spatial coherence of traditional singlemode continuous wave lasers produces a random intensity variation in projected patterns called speckle. This speckle phenomenon has historically made use of lasers in high-resolution lithography very difficult. The beam of excimer lasers is so highly multimode that speckles are, for all practical purposes, nonexistent in projected patterns. The application of excimer laser... 

One application of lasers to lithography is a maskless patterning technology. In this application, the spatial coherence of the laser is the essential property ... 

There are then no possibilities for the occurrence of irreversible radiationless decays in such small-molecule limit triatomics. However, interesting effects, arising from the coherent superposition of many levels, may still appear when (2.14) is violated. The presence of hyperfine structure makes this possibility very likely. For instance, Demtroder has observed nonexponential decays of excited states of NO2 in a molecular beam where the spacing between hyperfine levels is claimed to be sufficient to excite a single hyperfine component with his MHz bandwidth laser. Demtroder then has no recourse but to explain the nonexponential decays in terms of some elusive radiationless decay despite the fact that the conditions (2.2) for the small-molecule limit are obeyed and prohibit irreversible decays. It should, however, be recalled that when traveling along with the molecule in the molecular beam, the molecule encounters a pulse of radiation whose duration is given by the laser spatial extent divided by the molecular velocity. For a laser spot size of 10" cm and a molecular velocity of 10 cms -the pulse duration is 10 s. This yields an effective pulse frequency width of 10 MHz which could yield a coherent superposition of a number of hyperfine levels. The nonexponential decay of such a superposition is discussed in Section II. C. 

The key operational parameters of exciplex and excimer lasers used in optical lithographic applications include exposure-dose-related parameters comprising average power, pulse energy, repetition rate, and pulse width temporal coherence spatial coherence including beam dimensions, beam divergence, and beam uniformity and maintenance and reliability. Table 13.2 lists some of the key operational parameters of KrF, ArF, and F2 laser systems used in optical lithography. 

Exciplex and excimer lasers have very poor spatial coherence and therefore do not produce speckles in lithography. Speckles are the random interference patterns that result when, on illumination of an object with a spatially coherent wavefront, any... 

In addition to the various lamps, lasers can be used as light sources for absorbance detectors. The highly directional, spatially coherent emission from lasers can be efficiently collected and focused to a small spot within a capillary. These characteristics make lasers a viable choice for sources in absorbance detection systems (especially for smaller-diameter capillaries), although their use is less... 

The term laser is an acronym (light amplification by stimulated emission of radiation) that denotes a technical device operating on the basis of the stimulated emission of light. A laser emits monochromatic, spatially coherent, and strongly polarized light. The essential parts of a laser device are an active material and a resonator, i.e. an optical feedback (see Fig. 6.10). 

A famous example for the application of intensity-correlation interferometry in astronomy is the Hanbury Brown-Twiss interferometer sketched in Fig. 7.33. In its original form it was intended to measure the degree of spatial coherence of starlight (Vol. 1, Sect. 2.8) [945] from which diameters of stars could be determined. In its modern version it measures the degree of coherence and the photon statistics of laser radiation in the vicinity of the laser threshold [946]. 

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