POWER LASERS
8.3 POWERFUL GAS LASERS
8.3.2 COIL System
The COIL was developed by the military, starting in 1977, for airborne applications to replace the gas dynamic carbon dioxide laser (Section 8.3.1), which was used in the Airborne Laser Laboratory. COIL is used in the ABL and the ATL programs described in Section 12.2. Chemical lasers use a chemical reaction to pump the laser;
the large amount of pumping energy needed to pump an ultrahigh-power laser is carried efficiently in chemicals. COIL is over 15% efficient and uses iodine as the lasing material. Iodine emits 1.315m radiation, which is eye safe, carries well in an optical fiber, and couples well to most metals. It is often pumped with ultraviolet light, which breaks iodine molecules down by photolysis, leaving an energized iodine
POWERFUL GAS LASERS 161
FIGURE 8.9 Physical structure of the ALL gas dynamic CO2laser that shot down a missile in 1983.
atom for population inversion. In the case of COIL, chemically pumping iodine to an excited state is performed by excitation transfer pumping that involves two stages. In the first stage, oxygen is excited to a metastable state known as singlet delta oxygen, O∗2(1 ):.
O2−→O∗2(1 ) (8.33)
Then, in the second stage, the singlet delta oxygen is merged with iodine to transfer the excitation from the oxygen to the iodine molecule for population inversion of the iodine:.
O∗2+I−→O2+I∗ (8.34)
162 POWER LASERS
FIGURE 8.10 ALL gas dynamic CO2laser with fuel system and optics.
After pumping, lasing causes iodine to move from the excited stage to emit photons at 1.315m:
I2P1/2−→I2P3/2 (8.35)
We consider the two pumping stages separately: singlet delta oxygen production, equation (8.33), and excited iodine production, equation (8.34).
8.3.2.1 Singlet Delta Oxygen Production A lot of methods have been pro- posed for generation of singlet delta oxygen and many have been tested. The most convenient method for an airborne application, at left-hand side of Figure 8.11 [31], is to mix basic hydrogen peroxide (BHP) with chlorine, in which the word basic refers to the addition of a base such as KOH (or NaOH). The liquid BHP passes through as many as 12,000 orifices to create drops for a high surface area contact with the gas. The chlorine gas passes through an array of orifices and most of the chlorine is used up in the chemical reaction. The chemical reaction produces heat, potassium chloride, and oxygen in an excited state (singlet delta oxygen) with a spontaneous lifetime of 45 min. This operation occurs at left-hand side in the reactor as shown in Figure 8.11 [31] (from U.S. patent 6,072,820). Other methods for exciting (pump- ing) the oxygen are often simpler or faster but may be less efficient. For example, electric discharge, such as used for a CO2electric discharge laser EDL, is used in Ref. [146]. Another example claims to provide a laser pulse more quickly, useful in
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Chlorine gas
Basic hydrogen peroxide
Energized oxygen
Iodine with diluent enters nozzles
Energize oxygen generator
Laser light out
Transverse optical resonator
Diffuser slows up gas Reflectors
for cavity
FIGURE 8.11 Chemical oxygen–iodine laser.
an airborne weapon, by supersonic expansion of oxygen to produce a molecular beam that contains clusters of oxygen and singlet delta oxygen [19].
8.3.2.2 Excited Iodine Production for Lasing The singlet delta oxygen is now mixed with iodine in an array of nozzles in the second stage shown in Fig- ure 8.11 [31]. The singlet delta oxygen passes through interstitial spaces between nozzles that speeds it up, for example, from Mach 0.4 to Mach 1 (one times the speed of sound, which is 343 m/s at atmospheric pressure). The high-pressure iodine gas at, say, 100 psi is mixed with a nitrogen diluent (which does not participate in the chemical reaction) and the mixture is released through the aerodynamic nozzles that speed it up to Mach 5 (five times the speed of sound) and the pressure falls from 100 to 0.2 psi. Turbulent mixing is effective because fast moving iodine merges with slower moving singlet delta oxygen. The resulting output has a speed of Mach 3.5 and temperature of 100 K. This will cool the laser and lowers pressure for optimum gain in the laser. The laser beam is fixed at right angles to the gas flow by setting mirrors to form a cavity in this direction. Following the laser, expansion in a diffuser performs a function opposite to that of nozzles. It recovers the pressure from 0.2 to 3 psi from whence it can be pumped after scrubbing of toxic chemicals or absorbed for a closed system. Note that the nozzles are not used in the same way as in the CO2laser to produce population inversion because the chemical reaction provides the heat needed for thermal population inversion. However, the nozzles provide the temperature and pressure for optimum laser gain.
8.3.2.3 Absorbing the Waste Gases The hot waste gases can be used to detect the presence of an airplane and recognize that it is carrying a laser weapon. So, to remain undetected and for safety, the chemical oxygen–iodine laser must absorb all the hot nonatmospheric waste gases without exhausting them out of the plane.
A cryoabsorption vacuum pump is shown in Figure 8.12 [165] (from U.S. patent 6,154,478). The chemical oxygen–iodine laser is at the left. The waste gases pass
164 POWER LASERS
Nozzle Diffuser
Laser cavity Hydrogen
peroxide
Chlorine Nitrogen
Energized oxygen
generator Laser
beam out
Chemical oxygen iodine laser
Refrigerant
Vacuum vessel
Heat shield
Gas chiller
Zeolite bed
Clean exhaust gases
Cryosorption vacuum pump system
Filters remove chlorine and iodine Pumps
Isolation valve
FIGURE 8.12 Absorbing waste gases in COIL.
through a valve into the cryoabsorption vacuum pump in the right box. The gases pass into a chiller at 80 K, cooled by a refrigerant of liquid nitrogen or argon in a Dewar. The chiller condenses or freezes out chlorine gas, iodine, and water vapor, which are trapped on the cooled surfaces. Only cold and dry nitrogen and oxygen are left. These are absorbed by granulite zeolite in a vacuum vessel at 80 K. Absorption of the gases acts as a vacuum pump to draw the gases through the complete system until the zeolite bed fills and the pressure is no longer adequate. The valves are then controlled to regenerate the system and the pump filtered clean gases out from the chiller and zeolite bed at close to atmospheric pressure and temperature.