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Issue 3, May 2001

Discovery of the First and Only Laser in Outer Space

Courtney Peterson
Biology and Physics, Georgetown University
peterson@jyi.org


It happened on the last day that NASA's Kuiper Airborne Observatory (KAO) was supposed to fly before being permanently shut down. As the world's only airborne astronomical research facility, this unique observatory's 36-inch (91-cm) reflecting telescope allowed researchers to observe deep space from anywhere on Earth, well above the atmospheric cloud cover and water which blocks certain wavelengths of light. Several major discoveries had been made aboard the KAO, including the first sightings of the rings of Uranus and the definite identification of an atmosphere on Pluto. A lack of future funding, however, was forcing the KAO out of operation.

KAO

Vladimir Strelnitski, a researcher at the Smithsonian's National Air and Space Museum in Washington, D.C., hoped for precious observation time aboard the KAO. With only a few more days before the planned shutdown, Strelnitski flew to Hawaii, where the KAO was stationed. Although he was not scheduled to fly aboard the KAO, an informal promise had been made: weather permitting, he would be allowed to use the KAO before its final trip home to search for the elusive astronomical laser.

Strelnitski is an expert in astronomical masers, which are produced by the extreme conditions present in our Universe. A maser is the non-visible light equivalent of a laser (light amplification of stimulated emission of radiation). In both lasers and masers, a beam of light of a single wavelength is amplified (How is the light in masers and lasers amplified?). In a laser, visible or infrared light is amplified, whereas in a maser, microwave radiation is amplified. Microwave radiation has longer wavelengths than visible and infrared light, from 300 millimeters to 1/3rd of a millimeter, compared to 1/3rd to 1/3,000th of a millimeter for visible and infrared light. In both cases, the light produced comes from electrons emitting photons, or light, as they drop down into lower-energy orbits around nuclei (i.e. from electron transitions). Masers are very important because they can give researchers valuable information about the chemical and physical processes that occur in space. For example, masers can be used to probe the conditions of disks of gases and dust surrounding stars, from which planets may form.

Strelnitski was one of many astrophysicists working on astronomical maser theory. In 1995, he was tackling a thirty-year-old puzzle: why had no natural lasers been observed in space? Calculations had predicted the conditions necessary for a laser to form in clouds of ionized atomic hydrogen. Although scientists had observed thousands of strong masers with amplification factors of up to a trillion, no natural lasers had ever been found. Two possible lasers had been identified, but their amplifications were so low that the phenomena could easily have been ascribed to other processes such as fluorescence.

Strelnitski wanted to fly aboard the KAO to observe MWC 349A, a very bright, hot star (~ 30,000 times brighter than our Sun) in the constellation Cygnus. He had chosen this star because it is surrounded by a massive disk of gases and dust in which many masers formed by ionized hydrogen had been discovered, and because it is the largest known source of radio waves in the sky. He had first observed the star while looking for evidence of lasers about a year before; his target then had been the hydrogen transition called the H10alpha (52.5 mm) recombination line.

Although he found that the light from the H10alpha transition was bright enough to be a laser, Strelnitski could not state conclusively that the light was laser-induced. The results would probably not convince the scientific community; a larger amplification of the light beam was required. This time, aboard the KAO, Strelnitski wanted to observe the light produced by the hydrogen transitions H15alpha (169.4 mm) and H12alpha (88.8 mm).

On the morning of August 16, 1995, Strelnitski awaited word of whether he would be able to use the KAO's facilities. The weather that day was beautiful, in contrast to the past few days of rain. It appeared that bad weather would not prevent him from observing aboard the KAO.

Finally, Strelnitski received the good news: he would be able to use the KAO to observe these two transitions in the disk surrounding MWC 349A. A few hours later, he and the observing crew took off aboard the KAO. The weather that night was beautiful. Even if he didn't find a laser, as he looked out the window at the beautiful night sky Strelnitski knew it would be a remarkable moment for him, as one of the last people ever to fly aboard the KAO.

At 41,000 feet, the KAO's telescope was pointed at the source: MWC 349A. If there were any lasers in the disk surrounding the star, there would be no problem identifying them. The KAO's instruments were simply not sensitive enough to observe light produced by electron transitions in the absence of lasers. Slowly scanning the 169 mm energy band (H15alpha), there was nothing at first. But, then suddenly around 169.25 mm, there was a jump in the readings. Strelnitski and others aboard the KAO waited in anticipation. The next reading, taken at about 169.33 mm, was even higher. The tension and excitement aboard the KAO was mounting. If lasers were present around the star, the observed line profile should resemble a bell curve (why?), and should peak at 169.4 mm, the true wavelength of the H15alpha transition.

The next reading then, at about 169.42 mm, should be the highest yet, being close to the peak radiation intensity value. Five minutes later, the reading was even higher. Their data appeared to indicate that the observed H15alpha line was due to a laser. Strelnitski and his colleagues had discovered the first unmistakable laser in space!

The mood turned from tense to joyous aboard the KAO, and the researchers exchanged hugs, laughs and sighs of relief. After analysis, the H15alpha line was found to be about 5 standard deviations above the expected non-lasing level, with a calculated amplification of about 1000 - an undisputable positive identification. The H12alpha and H10alpha measurements were about 2-3 standard deviations above the expected level, a probable identification by astrophysical standards. Strelnitski knew that this would be big news in the astrophysical community.

One mystery remains: why are natural lasers so rare? In the paper Strelnitski published in the journal Science, he speculated on possible answers. According to the results obtained aboard the KAO, lasers can only form below a critical density of gas and dust - a density which is roughly a trillion times less dense than air on Earth. The critical density is, however, unique for each type of laser or maser. For lasers, this critical density is higher and can only be found in a very small region surrounding the star. This would explain why lasers are so hard to observe: the regions where they can form are very small and may be quite rare in stars. Even if they do form, they could be easily hidden by all the other light emitted by the star. (For more detailed information on why lasers are so rare, click here.)

Since Strelnitski and his colleagues' discovery of the first natural laser in space, no other lasers have been found. MWC 349A is an enormously large and bright star with many masers discovered in its disk prior to the discovery of the first laser. This is perhaps testimony to the rareness of the conditions required for natural laser formation. Whatever the case may be, it still remains a mystery why natural lasers are such an anomaly.

One last note: although the KAO is no longer in operation, the discovery of Strelnitski and colleagues helped obtain funding for the creation of a new airborne astronomical facility, the Stratospheric Observatory For Infrared Astronomy (SOFIA). SOFIA will begin operations in the year 2002.

 

Masers and Lasers: In Depth

How is the light in masers and lasers amplified, and what's this "amplification of stimulated emission of radiation"?

Intuitively, amplification implies that there is more of something at the end of some process than at the beginning. Light amplification means that we have more photons at the end of a process than at the beginning. How can we get more photons out than we put in? A process known as stimulated emission provides the mechanism. In stimulated emission, the electric field of an incoming photon causes a molecule in a higher energy level to emit a photon of the same wavelength and thus to drop to a lower energy level; the incoming photon is not affected in the process.

stimulated emission
The "stimulated" photon has the same wavelength and phase as the original photon and travels in the same direction. We now have our amplification mechanism: one incoming photon yields two outgoing photons. The problem now is how to get stimulated emission on a scale large enough to have amplification. If only a few molecules are in a higher energy state, then we cannot amplify light to observable levels partly because spontaneous emission will dominate over stimulated emission. Thus, we need to have more molecules in a higher energy state than in the lower the energy state.


How can we get enough photons in a higher energy level so that incident light can be amplified?

Let's take a population of molecules. If this population is in thermodynamic equilibrium, most molecules are in the ground state. In this situation, we say that very few of the upper levels are populated. "Upper level" refers to any level above the ground state, i.e. any excited state of the molecule.

But what would happen if this population of molecules was not in thermodynamic equilibrium? The upper levels would become significantly populated. When this happens, there is a chance for a population inversion to occur. In a population inversion, a higher energy level is more populated than another energy level that is lower in energy. If E3 > E2 > E1, and E2 is more populated than E1, then E2 is inverted relative to E1. If we can achieve population inversion, then we will have enough molecules in excited states for stimulated emission to produce observable amplification.
pump cycle


But how can E2 ever become more populated than E1?

Consider E1, E2 and E3 as forming a cycle. Molecules on energy level E1 are excited to energy level E3, then fall down to E2 through the emission of a photon, and finally return to E1 through the emission of a second photon. This cycle repeats itself over and over. Molecules, however, occupy a certain energy level only for a finite period of time that is characteristic of that energy level. The median "occupation" time is called the lifetime, t. If the lifetime, t3, of molecules on E3 is shorter than the lifetime, t2, of molecules on E2 (i.e. if t3 < t2), then more molecules will accumulate on E2 relative to E1. This process will invert level E2 relative to E1.

Additionally, if there are several pump states that feed into E2, this will help more molecules to accumulate on E2 relative to E1. In this scenario, E3 is called the pump or feeder state, and E2 is an intermediate, metastable state. The laser/maser photon is the "stimulated" photon, and the transition from E2 to E1 is called the working transition. A cycle with a pump state and at least one other lower energy level is called a pumping cycle. For a pumping cycle to form a population inversion, however, there must be at least three levels. If there are only two levels in the cycle, a population inversion can never be formed because, as Einstein proved, the probability of absorption is equal to the probability of stimulated emission for a given transition. Thus, the most pumping can achieve for a two-level system is to equalize the number of molecules on two levels. A third level, E3, provides the way for more molecules to accumulate in an excited state, E2, relative to the ground or lowest energy state, E1, by feeding molecules into E2.


But how can a population of molecules not be in thermodynamic equilibrium? i.e., What natural processes can cause lasers and masers to form?

The roundabout answer to this question is that a population of molecules will not be in thermodynamic equilibrium if the molecules have not had a chance to thermalize with the surrounding environment. According to Maxwell-Boltzmann principles, a population of molecules reaches thermodynamic equilibrium through collisions. If one part of the population has a different temperature than another part of the population, then through collisions, eventually the entire population will have the same temperature.

The key here is that a population of molecules will not reach thermodynamic equilibrium if the molecules do not collide enough. This translates into the following: below a critical density*, which is unique to the molecules and the transitions of the pump cycle, a population inversion can be maintained because collisions are not frequent enough to prevent the higher energy levels from becoming significantly populated. Some examples of processes that can create population inversions include: shock fronts (when particles are accelerated to supersonic speeds), stellar winds/flares, stellar radiation, collisions between populations of particles at different temperatures, and chemical reactions (the products formed are usually in metastable states.


Why does the observed profile line resemble a bell curve when lasers are present around a star?

The observed line profile should resemble a bell curve because of the Doppler Effect. Recall that for sound, the Doppler Effect says if a train is approaching an observer, its pitch is higher, but if the train is moving away from an observer, its pitch is lower. The same applies for light. For the side of a rotating disk that is "approaching" the observer, the light emitted appears to be higher in energy. For the side of the rotating disk that is moving away from the observer, the light appears to be lower in energy. Thus, rotation of the disk around the star causes a shift in energies of some of the emitted light. Instead of a single wavelength, a bell curve of many wavelengths surrounding the "true" wavelength is observed.


Why Might Astronomical Lasers Be So Rare?

Each recombination line has an optimum gas density for lasers or masers to form. The lower the principle quantum number, n, the higher the optimum density. Because the density in the disk surrounding MWC 349A falls off as R-3/2, where R is the distance to the center of the star, the lower n is, the smaller the region in which lasing can occur. The smaller the possible amplification region, the lower the intensity of the lasing since there is simply less material available for amplification. According to calculations, if the disk does not extend inward of 40 AU (roughly the distance from the Sun to Pluto) to MWC 349A, then the material in the disk is not dense enough to support detectable lasing by Ha lines with n < 10. Even if the disk extends inward of 40 AU or Astronomical Units (about the distance from our Sun to Pluto), because the lasing regions for n < 10 would be so small, the background radiation could easily become dominant over any lasing. Detection equipment with better resolution and higher sensitivity would thus be necessary to find these small laser "hot spots". Lastly, Strelnitski speculates that because any tilt in the disk would remove any small "hot spots" from view, it is likely that any detectable lasing could simply not be in our line of sight.


*Not all proposed pumping mechanisms require the population of molecules to be below a certain critical density, but these other mechanisms are rare or unproven.

 


 
Journal of Young Investigators. 2001. Volume Three.
Copyright © 2001 by Courtney Peterson and JYI. All rights reserved.
 
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