|MadSci Network: Physics|
Lasers sure do work underwater... unless the interior of the lasers is exposed to the water, which might cause the electronics to short circuit and/or the optics to get misaligned or contaminated. But if you were going to use a laser to do some underwater work, you might have to make some corrections for the beam propagation in the aqueous environment. Lasers have been used for underwater applications for quite some time. Probably the most widespread current use of lasers underwater is for transmission of telecommunications signals via fiber optics. The lasers used in such cases are generally semiconductor diode lasers operating at 800, 1300, or 1500 nanometer wavelengths -- all in the near-infrared portion of the spectrum. In this case, though, neither the lasers nor the fibers actually touch the water -- they're encased in waterproof cables and boxes that keep the very-corrosive seawater from degrading the optical elements. As a result, while these lasers operate under water, they're really not depending on or using anything associated with the water... so they're kind of "fake" underwater lasers. Another kind of laser system provides underwater sensing using a kind of laser radar: blue-green lasers located in airplanes are sometimes aimed so that they go straight down into water that the planes are flying over. Using very sensitive detectors in the airplane, the system detects the small amount of light reflected both from the surface of the water and the bottom of the harbor. This allows measurement of the water depth to accuracies of a foot or so. The big advantage of such systems is that they can collect data very quickly, because the airplane moves very quickly and the lasers can be pulsed at a high rate. One example of this is the use by Australian government planes of such a system to map their very large coastal areas -- it's much quicker and cheaper to use a laser radar to do this than it is to use conventional acoustic (sound wave) sonars to make such measurements from ships. Still another kind of laser system which is used underwater uses blue- green lasers to communicate from airplanes to submarines. As early as 1983, experiments were done which demonstrated that this could be done sucessfully -- but the systems were terribly expensive and you had to pretty much know where a submarine was to be able to talk to it. So it's unlikely that these systems will actually be deployed. Beyond this, there haven't really been too much uses of lasers underwater. Bob Ballard -- the man who mounted the expeditions that found the Titanic in the North Atlantic Ocean -- expressed some interest in trying to use blue-green lasers to assist in taking pictures of the Titanic, but found out that development of an appropriate system would be very expensive and not too much better than just using high power spotlights... which he ultimately used. The reasons that there aren't too many applications of lasers in an underwater environment is associated with the way light is transmitted underwater through 2 separate problems: 1. Most light is absorbed by water -- Water absorbs ultraviolet, yellow and red and infrared radiation very strongly, so that beams in these spectral regions cannot be transmitted very far -- meaning that systems using such lasers are pretty useless. On the other hand, water (seawater, that is) transmits blue-green light pretty well -- losing "only" about 5% of its original intensity for every meter it transmits through water. 2. There are often little specks of dust, tiny animals (phytoplankton), and tiny plants (photoplankton) in water, and these reflectd a little bit of the light, too, reducing intensity as it passes through water. This adds to the problems noted above in Item 1. The effect of these two effects is that lasers used underwater have to be much more powerful than those used in air, where absorption is generally much lower than in water. The lasers used in the surveying equipment that you are familiar with are probably near-infrared lasers emitting a few milliwatts of output power, modulated to allow easy range measurements. Lasers used for underwater optical work generally must emit output powers in the multi-megawatt (that's millions of watts!) range... and because of the large output power, they're only used in special configurations in which the outputs are very short pulses -- of a few billionths of a second duration -- of blue-green light. Lasers to be used for underwater applications are, as a result, very expensive and difficult to build. The bottom line is that lasers do work underwater, and have been used to do surveying of a sort. But the lasers are much different in color and power than the type with which you are familiar -- and much more expensive. They haven't been used too much, because of the cost and difficulty of building them -- particularly since other cheaper alternatives exist. As a result, it's unlikely that they'll be used extensively, except in very special circumstances. Thanks for the great question! Steve Guch
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