Re: Lasers (Combination of gases)?

Greetings:

During the first 30 years of the 20th Century QUANTUM MECHANICS was developed and theories were developed to explain the atomic behavior observed in nature much more accurately than could be explained by the simple orbiting electron model. Also, it was theorized that light consisted of PHOTONS, discrete packets of energy, that were quantized by frequency (color). This means that the light spectrum observed in a rainbow or from the sun passing through a prism is not continuous in color change but that each PHOTON has an exact frequency of oscillation (color) and the color changes are separated by one cycle per second. Also; red light PHOTONS have less energy in each photon with increasing photon energy in orange, yellow, green, blue, and violet PHOTONS.

PHOTONS of blue light have about 2 times more energy than photons of red light. This also means that their are billions of exact colors (notes of light) in the light spectrum just as a piano has about 88 notes on the key board. However, until the invention of the laser in 1960 these pure frequencies (notes or tones) of light could not be observed.

Scientists also theorized (also proven by experiment) that each electron surrounding an atom has an exact energy level (orbit) and that no two electrons in an atom can have the same exact energy. This quantizing of electron energy was pictured as atoms with electrons in hundreds of possible orbits with each orbit having an exact energy multiple of 1, 2, 3, 4 - - - times etc. from the lowest to the highest energy orbits.

Scientists also theorized (also proven by experiment) that if an electron jumps from a higher energy orbit to a lower energy orbit the atom gives off a PHOTON of light energy at a color that exactly equals the energy difference of the orbit energy change. Also, when an electron changes from a lower energy orbit to a higher energy orbit the atom absorbs light with photons that have the exact energy that matches the orbit energy change. Thus orbital energy changes could be used to control a light amplifier or oscillator.

In the 1950s scientists were looking for new types of QUANTUM MECHANICAL AMPLIFIERS for microwave radar and communications applications that would be much more sensitive than vacuum tube and transistor amplifiers then in use. In 1954 Professor Charles Townes and his students at Columbia University in New York City developed the world's first quantum mechanical amplifier and named it the MASER for (M)icrowave (A)mplification by (S)timulated (E)mission of (R)adiation. MASERS are the most sensitive amplifiers ever made by man and today they are installed in all of the giant antennas used for radio astronomy and for deep space communications.

The reason we can get pictures back from space craft visiting the outer planets of the solar system many millions of miles away is because of the giant antennas that collect the extremely small energy signals and the RUBY MASER amplifiers at the focal point of these antennas that are millions of times more sensitive than the amplifiers in our home electronic equipment. (MASERS and LASERS work in a similar yet basically different manner which I will not discuss here)

In the late 1950s scientists all over the world were trying to develop an OPTICAL MASER, a LIGHT AMPLIFIER. Dr. Maiman and his assistants at the Hughes Research Laboratories in Malibu, California made the first successful LASER demonstration in May 1960 using synthetic (man made) ruby crystals. The RUBY LASER produced a single pure frequency of deep red light just as quantum mechanics had predicted 50 years before. Maiman's ruby crystals were similar to the ruby crystals he was using in MASER amplifiers he was developing for microwave radar. Both Maiman's RUBY MASER and RUBY LASER are on display in the Smithsonian Institution Museum in Washington, D.C. There is a detailed drawing and photographs of Maiman's Ruby Laser at the following Web Site:

http://www.achilles.net/~jtalbot/history/ruby.html

We have discussed how electrons jumping between orbits either absorb or emit light photons depending on whether the electrons gain energy (absorb light) or lose energy (emit light). If we take a very pure gas, solid or liquid material (for example a ruby crystal) that has electron orbits spaced so that they could give off visible light (say red light) and then we add a fraction of a percent of special atoms called DOPANTS to the crystal (Chromium dopants in ruby), we can modify and control the electron orbits in the crystal. Thus we can make selected orbits much easier for the electrons to jump between. In the normal state the electrons in a material will fall to the lowest energy orbits possible. However, if we shine a great deal of light energy (for example from a flash lamp) into the material, most of the material's electrons will absorb light energy, temporarily placing the electrons in specific higher energy orbits. We do this in the RUBY LASER by surrounding the crystal with a very bright flash lamp similar to those used in photography. This is called OPTICAL PUMPING of the material and the unnatural distribution of electrons in higher energy orbits caused by the flash lamp is called a POPULATION INVERSION (the population of electrons in the higher energy orbit is much greater than the population of the electrons in the lower energy orbit).

Some lasers use electrical pumping, others use chemical reactions. Some huge lasers use smaller lasers to optically pump larger lasers. You can tour the giant NOVA laser facility at the following Web Site: http://ep.llnl.gov/nova/welcome.html

Now if a photon of light at the correct frequency passes through a material with a POPULATION INVERSION the photon will cause (stimulate) a large number of electrons to fall to the lower energy orbits at nearly the same time causing an amplification of the number of photons. This process is called STIMULATED EMISSION, the single photon stimulates many photons of the same color (frequency) to be emitted making a light amplifier.

Now that a light amplifier has been developed let's make it into an OSCILLATOR. We can't use wires for light feedback but we can use mirrors. Dr. Maiman coated each end of his ruby rod with mirrors so that the photons of light from STIMULATED EMISSION reflect thousands of times back and forth between the mirrors stimulating more and more electrons to emit more light. This is a light oscillator which is the most common form LASERS take. Now to get some useful light out of the device, Dr. Maiman put a small hole in the middle of one end mirror so that about one percent of the light in the ruby crystal could escape and form the first laser beam, while 99% of the light went on stimulating emission in the ruby crystal until all the electrons fell to the lower energy levels (Note: many reference books say that Maiman used a partially silvered mirror for the first laser, this is an error). The first RUBY LASER produced a pulse of laser light for only a microsecond, one pulse of light for each flash of the lamp. Later models used a continuous flash lamp and a continuous laser beam was obtained. Later many improvements were made in laser designs and the fields of LASER PHYSICS and LASER ENGINEERING were born.

The rest is history and hundreds of laser types have been invented. They operate at thousands of visible colors and in the infrared and ultraviolet spectrum. Each color of laser is set by engineering the electron orbits in the laser material. Laser materials used today include gasses, liquids and solid crystals.

You can see many new laser related activities at the Web Site for the Hughes Research Laboratories where the first laser was demonstrated:

Gas lasers were the first lasers two operate continuously rather than in pulses of energy and they are also engineered for optimized operation. The workhorse carbon dioxide (CO2) laser can be designed for output powers ranging from 1 watt to 10s of kilowatts and it is a good example to use to discuss your questions. CO2 lasers come in many shapes and sizes and they are used for welding and cutting in manufacturing plants and for surgery. CO2 lasers have also been used for experimental laser communications systems and for laser weapons. The CO2 laser has vibrational energy levels in the infrared rather than electron energy levels. That is the two oxygen atoms vibrate in different fixed patterns relative to the central carbon atom at each band of energy levels. The CO2 laser can operate at many wavelengths in the infrared; however a band of wavelengths around 10.6 micrometers are the wave lengths most often used for laser applications. A typical CO2 laser is made from a mixture of 3 gases; 3 parts helium molecules (He), 2 parts nitrogen molecules (N2) and 1 part carbon dioxide molecules (CO2). Although this group of gasses has many dozens of energy levels, I will simplify the discussion to bands of energy levels to help answer your questions.

The bands of vibrational energy levels for CO2 molecules in electron volts (eV), in order, above the ground state are around : (1) 0.08 eV, (2) 0.16 eV, (3) 0.17 eV , (4) 0.23 eV, (5) 0.25 eV, (6) 0.3 eV and (7) 0.35 eV. Nitrogen has only one vibrational energy level above the ground state in this region of energy at 0.3 eV and helium has no energy levels in the 0 to 4 eV energy region.

The 10.6 micrometer laser transitions, where the CO2 device is designed to operate uses the energy transition from level (6) to level (3). There is another 10.6 micrometer transition from level (7) to level (5); however, because of the nitrogen energy level, as discussed later, this transition is not usually used. The gas mixture is usually excited either by a high voltage discharge or by radio frequency energy exciting all of the dozens of the energy levels. As the CO2 molecules decay the population in the energy levels quickly drop toward the ground state and each transition such as (7) to (6) or (7) to (5) or (6) to (4) etc. emits photons generating many different wavelengths. This is called SPONTANEOUS EMISSION. Exciting all the energy levels at the same time does not leave enough energy in energy level (6) to transition to (5) to over come the light lost in the device and to make the device into a powerful laser oscillator.

Notice that nitrogen has only one energy level at 0.3eV matching CO2's energy level (6)! Thus all of the excited nitrogen molecules, and we have engineered it so that there are many more nitrogen than CO2 molecules, are excited to the same energy of level as level (6) of CO2. This is like having two tuning forks at the same frequency and energy is transferred from the N2 excited molecules in nitrogen energy level (1) as they fall to the ground state selectively to the CO2 molecules in the ground state exciting them in a resonance transfer of energy by collision to level (6). This results in the N2 atoms becoming a tuning mechanism and an efficient energy source to excite CO2 molecules only to level (6)! The energy level (1) of nitrogen is also called a METASTABLE level. Metastable levels can hold energy much longer than other levels greatly aiding the stimulated emissiom process.

The addition of nitrogen enables as much as 10% of the input energy to be converted to many watts of laser energy. In the first gas laser, the helium - neon laser, only one thousandth of one percent of the input energy is converted to laser light and these lasers can only generate a few milliwatts of laser power.

As the CO2 laser is operating spontaneous emission is heating up the gasses and partially filling all the energy levels including energy level (3) limiting the number of transitions that can occur from level (6) to (3). To help cool the gasses we add helium to the mixture which aids in conduction cooling of the discharge and helps depopulate all of the energy levels including level (3). Also, because helium does not have any energy levels below 4 eV it does not interact with the nice CO2 pumping effect we have designed with the nitrogen molecules.

Lasers that use laser pumps exactly matching the pump transition energy can be more than 50% efficient and this is the preferred method for super power lasers such as the NOVA laser referenced above.

Best regards, your Mad Scientist Adrian Popa

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Adrian E. Popa                  "Make everything as simple as
apopa@hrl.com             /FLASH   possible, but not simpler" 
(310)317-5209            /O/O/O/O/     -Albert Einstein 
FAX (310)317-5485 I-----/ / / / /---I                           
                  I((((/((RUBY)/))))))))))))))))))) MAY 1960  
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Hughes Research Labs    LAMP/         
Malibu, California 90265           The Birthplace of Ruby
http://www.hrl.com                 The World's First Laser
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