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M E A S U R I N G   L I G H T (continued)

7. Measuring Laser Radiation

Of course the principles for measuring light amplitude (as outlined in para. 4, para. 5, and para. 6 of this essay) are valid for the measurement of laser radiation, too. But this kind of light has some special properties that need special care at time of measurement. The next three paragraphs deal with three of these properties.

7.1 Monochromatism

Key words:

Laser radiation is very monochromatic. Though specalists bother about laser line structures, we may say that laser light contains just one wavelength. Where the special value of wavelength is defined by the type of laser.

On the one hand, this is an advantage:

a) Radiometry no longer needs BROAD-BAND FLAT DETECTORs. As shown in fig. 4.0-c of para. 4.0, spectral characteristics of the radiometer detector only need to be known. Since the radiation contains only one well-known wavelength, radiometer reading can easily (and even automatically) be corrected. An example of a laser power meter relying on this principle is shown in fig. 7.1.

    fig.7.1: radiometer for monochromatic sources (53 kByte)

b) Photometry no longer needs V(LAMBDA)-CORRECTED DETECTORs (para. 5.0). You see: the radiation contains only one well-known wavelength. For this wavelength (lambda)  there is just one product value
   V[(lambda)] * Km   ( that is,
'spectral luminous efficiency' times 'max.luminous efficacy of radiation' ).
For  V[(lambda)]  and for  Km  see example for para. 4.4.3 .
With this product value, radiometer reading can easily (and even automatically) be transformed into a photometric reading.

On the other hand, monochromatism is a challenge:

If somebody wishes spectrally resolved (para. 4.4) laser radiation measurement, in most cases he looks for extremely FINE SPECTRAL RESOLUTION. Laser linewidth is often no more given in nanometers, but in megahertz or even kilohertz!


Subject used in source
radiometer fig. 7.1 Rohde & Schwarz

Continued: 7.2 Coherence

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