Strontium Vapor Laser
Encyclopedia
A Strontium Vapor Laser is a laser
that produces at its output, high intensity pulsed light at a wavelength
of 430.5 nm in the blue-violet region of the visible spectrum
via vaporized Strontium
metal gas contained within a glass tube.
, Oxford by Deech and Sanders as early as 1968. Gain was measured over a 9 cm length of strontium vapor present in a 3 torr
buffer gas of helium or neon
and maintained at the correct temperature by an externally heated furnace. Three years later twelve further infra-red laser transitions in neutral strontium were reported by Cahuzac. Again the heat needed to provide sufficient vapor pressure was produced by external means. The tubes used here were 5-10mm in diameter and 75 cm in length. A 1.25m cavity was used with mirrors of approximately 98% reflectivity
.
In 1973, Latush and Sém from Rostov-on-Don State University, Russia
, observed visible laser action from the strontium vapor laser for the first time, at wavelengths of 430.5 nm and 416.2 nm. The active volume was contained in a ceramic tube 8mm in diameter and 60 cm long. Small pieces of strontium were placed inside the tube at equally spaced intervals and the necessary vapor pressure was produced by externally heating the assembley. Helium was used as the buffer gas, at pressures ranging from 2.5-35 torr
. Output power was found to increase with increasing buffer gas pressure.
laser is excited by a high current, pulsed electrical discharge. The gain medium consists of a small amount of strontium vapor held in a relatively high pressure buffer gas
of helium
. Average gas temperatures are in the region of 800°C.
A capacitor
, charged to several tens of kilovolts, is repetitively discharged through the gas mixture. During each discharge pulse through the laser medium the neutral strontium vapor is ionized to Sr2+ as the electrons in the outer shell are removed, while only a small fraction of the helium buffer gas is ionized due to its greater ionization potential
. On termination of the current pulse, rapid cooling of the electron
s occurs, permitting three-body electron-electron-Sr2+ collisions to occur to form the most
highly excited states of Sr+, as shown:
The excess kinetic energy
evolved in this process is carried away by the third body, an electron. De-excitation of the high-lying energy levels of Sr+ then occurs, due to collisions with the remaining free electrons in the plasma
. This cascade of recombined electrons, down the Sr+ energy levels, continues freely until the 62S level is reached. The downward transition across the relatively large energy gap, 62S-52P, acts as a bottleneck for the electron de-excitation process, which generally proceeds faster for closely spaced levels. A population inversion
therefore builds up in the 62S1/2 upper laser level. Inversion occurs between this and the 52P3/2 lower laser level, which is cleared to the metastable and ground levels also by collisions with electrons.
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
that produces at its output, high intensity pulsed light at a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of 430.5 nm in the blue-violet region of the visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
via vaporized Strontium
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
metal gas contained within a glass tube.
History
Laser action on two of the infra-red transitions in Sr+ was first discovered in the Clarendon LaboratoryClarendon Laboratory
The Clarendon Laboratory, located on Parks Road with the Science Area in Oxford, England , is part of the Physics Department at Oxford University...
, Oxford by Deech and Sanders as early as 1968. Gain was measured over a 9 cm length of strontium vapor present in a 3 torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
buffer gas of helium or neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
and maintained at the correct temperature by an externally heated furnace. Three years later twelve further infra-red laser transitions in neutral strontium were reported by Cahuzac. Again the heat needed to provide sufficient vapor pressure was produced by external means. The tubes used here were 5-10mm in diameter and 75 cm in length. A 1.25m cavity was used with mirrors of approximately 98% reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
.
In 1973, Latush and Sém from Rostov-on-Don State University, Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
, observed visible laser action from the strontium vapor laser for the first time, at wavelengths of 430.5 nm and 416.2 nm. The active volume was contained in a ceramic tube 8mm in diameter and 60 cm long. Small pieces of strontium were placed inside the tube at equally spaced intervals and the necessary vapor pressure was produced by externally heating the assembley. Helium was used as the buffer gas, at pressures ranging from 2.5-35 torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
. Output power was found to increase with increasing buffer gas pressure.
Population Inversion Mechanism
The strontiumStrontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
laser is excited by a high current, pulsed electrical discharge. The gain medium consists of a small amount of strontium vapor held in a relatively high pressure buffer gas
Buffer gas
A buffer gas is an inert or nonflammable gas. In the Earth's atmosphere, nitrogen acts as a buffer gas. A buffer gas adds pressure to a system and controls the speed of combustion with any oxygen present...
of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
. Average gas temperatures are in the region of 800°C.
A capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
, charged to several tens of kilovolts, is repetitively discharged through the gas mixture. During each discharge pulse through the laser medium the neutral strontium vapor is ionized to Sr2+ as the electrons in the outer shell are removed, while only a small fraction of the helium buffer gas is ionized due to its greater ionization potential
Ionization potential
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
. On termination of the current pulse, rapid cooling of the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s occurs, permitting three-body electron-electron-Sr2+ collisions to occur to form the most
highly excited states of Sr+, as shown:
- Sr2+ + 2e− → Sr+* + e− + K.E.
The excess kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
evolved in this process is carried away by the third body, an electron. De-excitation of the high-lying energy levels of Sr+ then occurs, due to collisions with the remaining free electrons in the plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
. This cascade of recombined electrons, down the Sr+ energy levels, continues freely until the 62S level is reached. The downward transition across the relatively large energy gap, 62S-52P, acts as a bottleneck for the electron de-excitation process, which generally proceeds faster for closely spaced levels. A population inversion
Population inversion
In physics, specifically statistical mechanics, a population inversion occurs when a system exists in state with more members in an excited state than in lower energy states...
therefore builds up in the 62S1/2 upper laser level. Inversion occurs between this and the 52P3/2 lower laser level, which is cleared to the metastable and ground levels also by collisions with electrons.