MadSci Network: Chemistry
Query:

Re: Lasers passing through non-flammable liquids

Date: Sat Aug 28 04:46:26 1999
Posted By: Steve Guch, Post-doc/Fellow, Physics (Electro-Optics/Lasers), Litton Systems, Inc., Laser Systems Division
Area of science: Chemistry
ID: 933252605.Ch
Message:

As is the case with many science questions, the answer is "... it 
depends."  I'd like to generalize the question a little to include the 
possibility that the laser might pass through a flammable liquid, too.

In general, if a laser "passes through" a liquid with little or no 
absorption or loss of intensity, nothing much happens.  The beam is likely 
to be bent a little as it goes into and emerges from the liquid, due to 
the fact that the liquid refractive index will probably be somewhere 
between 1.3 and 1.6.  And the position at which the beam focuses will 
change, too, for the same reason.  But if the laser doesn't lose much 
intensity, that's probably about it...

On the other hand, if the laser interacts strongly with the liquid a 
tremendous number of phenomena -- many weird and wonderful -- can occur, 
only a few of which I've listed below:

If the beam is absorbed, the liquid will be heated and it will exhibit 
changes in refractive index due to the density gradients of the different 
temperature regions of the liquid.  If you tried to look through the 
liquid, it would look "all wavy" and the beam emerging from the liquid -- 
if any emerges at al -- will also be "all wavy."  If the beam is pretty 
high power and strongly absorbed, the liquid may even boil -- and could 
even form a steam bubble which is generated so rapidly that it looks like 
an explosion.

More interesting things occur if the beam isn't actually absorbed, but 
interacts with the liquid through what are called non-linear processes.  
In the case of Raman scattering in a liquid made up of molecules which 
have several atoms bound together in their structure, a small fraction of 
the beam is converted to light which emerges at all angles (not just along 
the beam direction) but with a wavelength which is shifted by an amount 
which depends on the particular molecule involved.  This is impossible to 
see under normal conditions, because the conversion efficiency is so 
small.  However, if you use a lens to focus the beam into the liquid to 
increase the intensity, you can create Stimulated Raman Scattering (SRS)
which produces a new beam (not just scattered radiation) at the shifted 
wavelength -- and you can convert 40 or 50% of the input power to the new 
wavelength!  For example, if you put a focused beam of 0.53 micron green 
light from a frequency-doubled neodymium YAG laser through benzene or 
cyclohexane (both very flammable!), the output beam has both residual 
green and red light beams in it.  And, if you focus still further in these 
liquids, a process called Stimulated Brillouin Scattering takes over and 
forms a mirror that reflects the input beam back on itself exactly -- not 
like a mirror, but in a way that causes the focusing beam to diverge into 
the lens that focuses it so that it's exactly reversed, in a phenomenon 
known as phase conjugation that looks a lot like "time reversal" (but it 
isn't).  This SBS process can also be 30% or so efficient, and results 
from the interaction of sound waves generated in the liquid with the beam 
that created them.  The SRS and SBS processes, and others like them, have 
been used in the past for a number of useful purposes.  SRS has been used 
to convert the very non-eyesafe output of a 1.06 micron neodymium YAG 
laser to 1.54 microns -- which is relatively safe to observe at relatively 
high powers.  SBS has been used to clean up the output of high power laser 
1-pass amplifiers by using the process to "undo" the thermal perturbations 
imposed on the beam being amplified in the first pass by doing phase 
conjugation and picking off the final amplified beam after its return 
reflection through the amplifier.

Interestingly, I haven't said much about flammability yet, because it's 
largely irrelevant.  If you focus any really powerful (e.g., multi-
megawatt in either short or long pulse) beam hard enough -- into a small 
enough spot -- either the temperature will rise to a very high value 
(e.g., 1000's of C) or the localized electrical fields due to the beam 
will get so high that "something bad" happens.  The bad thing is generally 
boiling and/or decomposition of the material.  If this occurs deep in the 
body of a large volume of an unconstrained liquid, the result is usually 
pretty non-catastropic:  a gas bubble of some sort forms, rises to the 
surface, and pops.  If it's big and the fluid is nasty, it can make a mess 
and be hazardous as the dickens if it pops in an inhabited area.  With 
some liquids, I suppose it's possible that an explosive, exothermic 
detonation could occur -- but I can't think of any offhand.  More likely, 
it you have the liquid in a closed or semi-closed container, the shock 
wave or pressure wave produced by the boiling or decomposition could break 
the container, exposing the high temperature fluid to air.  In this case, 
if the fluid is flammable, the whole thing could go up in a raging fire if 
the liquid is above its flash point -- in fact, I did this once with a 
sealed cell of benzene and made an absolute mess of my lab with raging 
flames and hideous black smoke filling the room above my optics bench:  
stupid, embarrassing, dangerous.  Alternatively, the broken container 
could release vapors of the flammable liquid into an area where the 
focused laser beam exists, again resulting in fire as the vapor suffers an 
ignition spark in the air/liquid mix soon after the mix is created:  very 
bad, as I can attest from experience.  Interestingly, these horrible fires 
occur in liquids which are quite flammable but which are also generally 
quite transparent to the laser light -- only when the beam is focused so 
that it decomposes the liquid or generates high temperatures in the 
extremely highly localized power (albeit slightly absorbed) focal region 
do bad things occur.

This answer goes a little afield from your question, but I hope that it 
provides you with some view as to the complexities of light/matter 
interaction.  I've been working with this lasers for about 25 years and 
have only begun to scratch the surface of the phenomena involved.

Before I close, however, I'd like to add a note about lab safety.  Unless 
you are using a very low power laser -- e.g., Class 1, usually under 1 mw -
- it's mandatory that you use extreme care to avoid potential eye damage.  
Also, use of any liquid chemicals other than water should be approached 
with extreme caution, since many are toxic or carcinogenic -- obtain and 
consult the Material Safety Data Sheet (MSDS) associated with any 
materials you may be thinking about using and talk with a good chemist 
before you actually obtain the materials, to avoid difficulties such as 
the ones I referenced above.


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