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

7. Measuring Laser Radiation (continued)

7.3 Power Range

Key words:

Range of occuring powers:

Laser radiation conveyes power like any kind of light does. But amount and density of laser power differs by far more than you know from usual light applications.

a) At the lower end, consider the receiving end of a fiberoptic telecommunications line:
received energy per impulse will be near the noise limit, probably about 100 photons.
With photon energy of
   E(photon) = h * (ny)     from para. 3

and at infrared wavelengths of about
    (lambda) = 1.5 E-6 m ,

which translate to infrared frequencies of about
   (ny) = (3 E8 m/s) / (1.5 E-6 m) = 2 E14 Hz ,

the energy of one received 100-photon-impulse will be:
     E(100photons) = 100 * 6.626 E-34 W*s^2 * 2 E14 s^(-1) = 13 E-18 W*s
     E = 13 Atto-Joule!

Even at pulse repetition rates in the 100-GHz-domain, this represents a faint mean power of
     P = 13 E-18 W*s * 100 E9 s^(-1) = 1300 E-9 W
     P = 0.0013 mW

b) At the upper end, you'll see huge power and energy densities. The reason is, laser light can readily be amplified, collimated, and focussed. Kilowatt laser welding machines are eye-catching (hopefully not eye-destroying) events. And nuclear laser fusion experiments even need huge  2 Mega-Joules  of laser energy per shot (1).
This fusion-igniting energy is to be transferred in a mere  10 ns  , which makes an instantaneous power of
     P = 2E6 W*s / 10E-9 s
     P = 200E12 W

Power meters' measurement ranges:

Between SENSITIVITY (or at least resolution) of a power meter and its DAMAGE THRESHOLD there is a factor between 10^4 and 10^9, depending on instrument type.

These ranges are extended beyond the usual 10^6 of good (silicon-)photo diodes by using various ATTENUATION devices: iris diaphragms, grey filters, beam splitters of different beam power ratios, and the like.

Laser power meters often show extremely sturdy detector heads  --  obviously designed for high-power application.

When selecting a laser power meter, watch out that you get one with a power range that fits to your needs.

Meter examples:

Figures 7.3-a, 7.3-b, and 7.3-c show a collection of internet-available laser power meter images; but I'm sure you'll find another dozen if you start a search.

fig.7.3-a: laser power meter by Kentek (31 kByte)

fig.7.3-b: laser power meter by New Focus / Edmund Optics (18 kByte)

fig.7.3-c: various detectors by OPHIR (110 kByte)

Link List and Literature

Subject used in source
Laser energy for NIF project text ref. (1) Lawrence Livermore National Laboratory
Low cost laser power meter fig. 7.3-a Kentek
Compact laser power meter fig. 7.3-b NEW FOCUS
Compact laser power meter fig. 7.3-b Edmund Optics
Laser power meter detector heads fig. 7.3-c Ophir

Continued: 8. Comparison Tables

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Last modified April 29th 2004 23:14