Coherent light generators – Particular active media – Gas
Patent
1998-11-30
2000-08-29
Scott, Jr., Leon
Coherent light generators
Particular active media
Gas
372 3, 372 98, H01S 322
Patent
active
061119059
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of purifying gas in a Raman cell. More particularly, it is related to a method of purifying methane gas in a Raman cell by making the gas pass through an absorber. The invention also relates to a Raman cell that includes an absorber.
STATE OF THE ART
In some laser applications it can be desirable to change the frequency of the laser light. One example is when making distance measurements by means of laser where it is desirable to use laser light having such a frequency that is not harmful to the human eye. For this application also a high pulse power and a good transmission in the atmosphere are required. Laser light having wavelengths around 1.5 .mu.m have proved to fulfil these requirements.
Raman cells are often used for shifting the frequency of laser light. A Raman cell contains a gaseous Raman active medium. A pumped laser emits short laser pulses, so called laser shots, having a high output power, which are focused by means of a lens system to the center of the Raman cell. Owing to the resulting strong electromagnetic field the electron states of the gas molecules are excited from an initial level to a virtual energy level. Returns thereafter occur to a final level having a higher energy than the initial level, whereby laser light having a longer wavelength than the wavelength of the pumped laser is emitted. The difference between the pumped laser wavelength and the emitted shifted wavelength is called a Raman shift. The obtained Raman shifted wavelength depends on the Raman medium used and the pumped laser used. Hydrogen gas and methane gas are examples of gases having Raman shifts.
In order to obtain the above desired wavelength that is secure to the eyes and is about 1.5 .mu.m, for the pumped laser a Nd:YAG laser can be used combined with a Raman cell filled with methane gas. The wavelength after passing the pumped laser beam that has a wavelength of 1.064 .mu.m through the methane gas is then 1.542 .mu.m. Raman cells filled with methane gas are also used in other applications than that described above and can also be used combined with other pumped laser sources which then result in different wavelengths. The use of methane gas as the Raman medium has many advantages. Methane gas has a large cross-section for scattering, a good amplification, is a stabile compound and has ample supply.
In order that the Raman process that results in the frequency shift will have a good efficiency, the beam of the pumped laser must be focused in the cell, whereby the intensity of the beam in the center of the cell will be very high. When the pumped laser beam is focused in the methane gas such a high field strength is obtained that the probability of the methane molecule being broken cannot be neglected. The process which then takes place is that a hydrogen atom is torn away from the methane molecule whereby methyl radicals and hydrogen gas are obtained according to the following:
The methyl radicals which are formed in the initial stage of the process according to the above are extremely reactive and can react with each other so that long chains of hydrocarbon are formed. By other processes like plasma-induced polymerization organic products are formed which by the laser radiation are burnt to stick to the inner side of the input and output windows of the Raman cell, exactly at those places where the laser beam passes. This process is developed slowly and degrades the cell at the rate of the number of laser shots. Depositions of this kind on Raman windows is a known problem and different solutions to this problem have previously been proposed.
From the U.S. Pat. No. 4,751,714 a Raman cell is proposed in which hydrogen gas is supplied to the methane gas in such a quantity that a balance between the decomposition of methane gas and the creation of methane gas is set according to the following:
By maintaining this balance deposition of carbon is prevented at the surface of the Raman window. The disadvantage of this method is that the depositions do not c
REFERENCES:
patent: 4723254 (1988-02-01), Turner
Jr. Leon Scott
SAAB Dynamics Aktiebolag
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