Plasma assisted oxygen-iodine laser

Details Plasma assisted oxygen-iodine laser Plasma assisted oxygen-iodine laser

2002-11-14

2004-02-10

Ip, Paul (Department: 2828)

Coherent light generators

Particular pumping means

Chemical

C372S055000, C372S022000

Reexamination Certificate

active

06690707

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to the field of lasers. More particularly, it pertains to a method and apparatus to perform the method of using electrical energy-generated plasma in a chemical oxygen-iodine laser (known as a “COIL”) to reduce the loss of energy generally encountered in the dissociation process of I
2
that inherently robs energy from interaction with oxygen in the singlet delta state.
2. Description of the Prior Art
Lasers (an acronym for Light Amplification by Stimulated Emission of Radiation) are devices producing intense, coherent and typically highly directional beams of light. Lasers are based upon the fundamental process of stimulated emission, a process by which a collection of atoms, molecules, ions or other medium actually amplifies light energy. This amplification occurs due to an interaction in which the light field interacting with the excited atoms induces them to radiate light precisely in phase with the optical input signal being amplified. The condition under which light amplification can occur is referred to as a “population inversion”. This description broadly applies to a condition in which the excited population density exceeds that of a lower level. This population inversion can be created through numerous pumping processes, chemical, optical, electrical, and others. An inverted population amplifies spontaneous emission leading to the effects of stimulated emission, light amplification and oscillation, sort of chain or domino effect, by which one atom's emission of light quanta (photon) induces subsequent atoms to emit at the same frequency, in phase with and contributing to the initial optical emission. This process is the basic process of stimulated emission and is in general necessary to all laser systems.
Lasers (as an engineering device), in general, consist of three major sub-systems, (1)the light amplifying medium (gain medium), (2) the energy/power supply or “pump source”, “pumping process” or laser pump mechanism, that introduces energy into the gain medium and allows for light amplification, and (3) the optical resonator, or optical cavity laser that allows for laser oscillation to build up and a coherent optical laser beam to be extracted for various applications. Lasers have multiple variations on the type of gain media, the energy source and the optical resonator system.
All lasers require some form of energy to pump or excite the gain medium and create the population inversion needed for laser action (called “lasing”). Chemical lasers directly utilize energy released by a gas phase chemical reaction to create the atomic or molecular population inversion needed for a laser device.
Atomic iodine lasers were demonstrated relatively early in the era of laser technology in about 1964. The earlier atomic iodine lasers were characterized by near infrared emission and generally only in brief pulses. These lasers were usually pumped through an optical process involving intense flash-lamps to dissociate an iodine compound and lead to the excited iodine. In general, these lasers were pulsed and were of interest for research involving short duration, very powerful optical pulses for studying laser/matter interactions at high intensity. Beginning in the late 1970's the consideration was given for chemical reaction schemes for producing the excited atomic iodine laser state for high average power, military class lasers (high energy laser “HEL” weapons).
A particular chemical laser is the chemical oxygen iodine laser or simply the “oxygen-iodine” laser. This system is complex, and requires some explanation of the overall process. Chemical energy is utilized in a complex reaction between an excited state of molecular oxygen and atomic iodine. The excited state of molecular oxygen is called a “singlet delta state” of oxygen and is designated in molecular spectroscopic terms as “O
2
(a
1
&Dgr;)”. O
2
(a
1
&Dgr;) may be produced by a wide variety of reactions. A common production reaction is that between basic hydrogen peroxide (hydrogen peroxide with an alkaline hydroxide, such as potassium hydroxide, added) and chlorine gas. There are other known methods of producing the excited state of molecular oxygen O
2
(a
1
&Dgr;) that include chemical, electrical and optical means and hybrids of them. This invention does not require any specific means of producing singlet delta state oxygen.
There are other gasses that may be combined with atomic iodine in a gas laser. One such gas is the excited state of molecular nitrogen chloride, known as “singlet nitrogen chloride” and is designated in molecular spectroscopic terms as “NCl(a
1
&Dgr;)”. This singlet nitrogen chloride is also useful in the instant invention.
The optimal energy transfer partner atom is atomic iodine. This excited state is not, by itself, suited to laser operation. However, it can be mixed with another gas, and through collisions transfer it's excitation energy to an atom suited for laser action. Such a scheme is called “collisional resonant energy transfer”. Such lasers use an excited energy “donor” species to transfer energy (through molecular to atomic collision) to a receptor species (the actual lasing species).
The chemical oxygen-iodine laser (known as a “COIL”) was first demonstrated as a chemical laser system in 1978-79. The laser showed promise for scaling to high average powers. However, a problem with the COIL system was recognized at this early development phase.
The iodine atom must first be generated by the dissociation of molecular iodine vapor. This is done in conventional COIL systems by injecting I
2
vapor into a flow of O
2
(a
1
&Dgr;). In a complex and not fully understood process, anywhere from four to seven separate O
2
(a
1
&Dgr;) excited molecules sequentially collide with the I
2
, each depositing excitation energy before the I
2
chemical bond is finally overcome, and free iodine produced. This process, while providing atomic iodine needed for lasing, also very significantly depletes the energy stored in the O
2
(a
1
&Dgr;) excited state flow. Energy depleted from the O
2
(a
1
&Dgr;) population and flow field results in a loss in laser power. Estimates are that up to 15% of the chemical energy potentially available for high power laser output is lost by this inefficient iodine dissociation step to generate free iodine atoms. The remaining portion of un-depleted O
2
(a
1
&Dgr;) is then used to actually pump iodine atoms on the 1.315 micron, near infrared, laser transition. A micron=1 micrometer or 1 millionth of a meter and is a standard unit of optical wavelength).
The conventional COIL system is typically configured as a supersonic flow laser. Subsonic flow versions of COIL devices have also been demonstrated. In this configuration, helium, or other inert and O
2
(a
1
&Dgr;) are expanded from a higher pressure “plenum” through a supersonic nozzle. Molecular iodine vapor is injected at or near the nozzle throat and the conventional mixing/dissociation process occurs. Subsonic flow versions of COIL devices have also been demonstrated. In conventional existing COIL systems the design uses a helium carrier gas for the chemically generated excited O
2
(a
1
&Dgr;) state. This mixture of helium and excited oxygen is transported to the nozzle or throat of a supersonic expansion nozzle. Molecular iodine is injected into the expanding O
2
(a
1
&Dgr;)+ helium flow at or near the nozzle throat. This mixing results in a complex multi-step reaction by which four to seven separate O
2
(a
1
&Dgr;) molecules are required to break up one iodine molecule of I
2
. This robs the COIL system of useful O
2
(a
1
&Dgr;) to drive the laser process with iodine atoms. As a result the energy and power output of a COIL device can be decreased by a substantial amount (about 15%) compared with what should be possible, were all of the O
2
(a
1
&Dgr;) made available for powering iodine atom laser action.
Further, the mixing and injection of the molecular iodine into the expanding O
2
(a
1
&Dgr;) and helium gas flow re

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