The purpose of the work is to study the main characteristics and parameters of a gas laser, in which a mixture of helium and neon gases is used as an active substance.
3.1. Operating principle of helium-neon laser
The He-Ne laser is the typical and most common gas laser. It belongs to atomic gas lasers and its active medium is a mixture of neutral (non-ionized) atoms of inert gases - helium and neon. Neon is a working gas, and transitions occur between its energy levels with the emission of coherent electromagnetic radiation. Helium plays the role of an auxiliary gas and contributes to the excitation of neon and the creation of a population inversion in it.
To start lasing in any laser, two most important conditions must be met:
1. There must be a population inversion between the working laser levels.
2. The gain in the active medium must exceed all losses in the laser, including “useful” losses for radiation output.
If there are two levels in the system E 1 And E 2 with the number of particles on each of them respectively N 1 And N 2 and degree of degeneracy g 1 And g 2, then population inversion will occur when the population N 2 /g 2 upper levels E 2 there will be more population N 1 /g 1 lower level E 1, that is, the degree of inversion Δ N will be positive:
If the levels E 1 And E 2 are non-degenerate, then for inversion to occur it is necessary that the number of particles N 2 on the top level E 2 was more than the number of particles N 1 at the lower level E 1 . Levels between which the formation of population inversion and the occurrence of forced transitions with the emission of coherent electromagnetic radiation are called working laser levels.
The population inversion state is created using pumping– excitation of gas atoms by various methods. Due to the energy of an external source called pump source, Ne atom from the ground energy level E 0, corresponding to the state of thermodynamic equilibrium, goes into the excited state Ne*. Transitions can occur to different energy levels depending on the pumping intensity. Next, spontaneous or forced transitions to lower energy levels occur.
In most cases there is no need to consider all possible transitions between all states in the system. This makes it possible to talk about two-, three- and four-level laser operating schemes. The type of laser operating circuit is determined by the properties of the active medium, as well as the pumping method used.
The helium-neon laser operates according to a three-level scheme, as shown in Fig. 3.1. In this case, the pumping and radiation generation channels are partially separated. Pumping of the active substance causes transitions from the ground level E 0 to excited level E 2, which leads to the occurrence of population inversion between the operating levels E 2 and E 1 . An active medium in a state with population inversion of operating levels is capable of amplifying electromagnetic radiation with a frequency 
due to stimulated emission processes.
Rice. 3.1. Diagram of energy levels of the working and auxiliary gas, explaining the operation of a helium-neon laser
Since the broadening of energy levels in gases is small and there are no broad absorption bands, obtaining population inversion using optical radiation is difficult. However, other pumping methods are possible in gases: direct electronic excitation and resonant energy transfer during collisions of atoms. The excitation of atoms in collisions with electrons can most easily be carried out in an electric discharge, where electrons accelerated by an electric field 
can acquire significant kinetic energy. During inelastic collisions of electrons with atoms, the latter go into an excited state E 2:
It is important that process (3.4) is resonant in nature: the probability of energy transfer will be maximum if the excited energy states of different atoms coincide, that is, they are in resonance.
The energy levels of He and Ne and the main operational transitions are shown in detail in Fig. 3.2. Transitions corresponding to inelastic interactions of gas atoms with fast electrons (3.2) and (3.3) are shown with dotted upward arrows. As a result of electron impact, helium atoms are excited to the levels 2 1 S 0 and 2 3 S 1, which are metastable. Radiative transitions in helium to the ground state 1 S 0 are prohibited by selection rules. When excited He atoms collide with Ne atoms located in the ground state 1 S 0, excitation transfer (3.4) is possible, and neon goes to one of the 2S or 3S levels. In this case, the resonance condition is satisfied, since the energy gaps between the ground and excited states in the auxiliary and working gas are close to each other.
Radiative transitions can occur from the 2S and 3S levels of neon to the 2P and 3P levels. The P levels are less populated than the upper S levels, since there is no direct transfer of energy from He atoms to these levels. In addition, the P levels have a short lifetime, and the nonradiative transition P → 1S devastates the P levels. Thus, a situation arises (3.1), when the population of the upper S levels is higher than the population of the underlying P levels, i.e., between the S and P levels a population inversion, which means transitions between them can be used for laser generation.
Since the number of S and P levels is large, a large set of different quantum transitions between them is possible. In particular, from four 2S levels to ten 2P levels, the selection rules allow 30 different transitions, most of which generate lasing. The strongest emission line during 2S→2P transitions is the line at 1.1523 μm (infrared region of the spectrum). For the 3S→2P transitions, the most significant line is 0.6328 μm (red region), and for 3S→3P – 3.3913 μm (IR region). Spontaneous emission occurs at all listed wavelengths.
Rice. 3.2. Energy levels of helium and neon atoms and operating diagram of a He-Ne laser
As stated earlier, after radiative transitions to P levels, nonradiative radiative decay occurs during transitions P→1S. Unfortunately, the 1S levels of neon are metastable, and if the gas mixture does not contain other impurities, then the only way for neon atoms to transition to the ground state from the 1S level is through collision with the walls of the vessel. For this reason, the system gain increases as the diameter of the discharge tube decreases. Since the states of 1S neon are emptied slowly, the Ne atoms are retained in these states, which is very undesirable and determines a number of features of this laser. In particular, when the pump current increases above the threshold value j pores there is a rapid increase, and then saturation and even a decrease in the laser radiation power, which is precisely explained by the accumulation of working particles at the 1S levels and then their transfer to the 2P or 3P states when colliding with electrons. This does not make it possible to obtain high output radiation powers.
The occurrence of population inversion depends on the pressure of He and Ne in the mixture and the temperature of the electrons. The optimal gas pressure values are 133 Pa for He and 13 Pa for Ne. The electron temperature is set by the voltage applied to the gas mixture. Typically this voltage is maintained at a level of 2...3 kV.
To obtain laser lasing, it is necessary that positive feedback exist in the laser, otherwise the device will only work as an amplifier. To do this, the active gas medium is placed in an optical resonator. In addition to creating feedback, the resonator is used to select types of oscillations and select the lasing wavelength, for which special selective mirrors are used.
At pump levels close to the threshold, lasing using one type of oscillation is relatively easy. As the excitation level increases, unless special measures are taken, a number of other modes arise. In this case, generation occurs at frequencies close to the resonant frequencies of the resonator, which are contained within the width of the atomic line. In the case of axial types of oscillations (TEM 00 mode), the frequency distance between adjacent maxima 
, Where L– length of the resonator. As a result of the simultaneous presence of several modes in the radiation spectrum, beats and inhomogeneities arise. If only axial modes existed, then the spectrum would consist of separate lines, the distance between which would be equal to c
/
2L. But in the resonator it is also possible to excite non-axial types of oscillations, for example TEM 10 modes, the presence of which strongly depends on the configuration of the mirrors. Therefore, additional satellite lines appear in the radiation spectrum, located symmetrically in frequency on both sides of the axial types of oscillations. The emergence of new types of oscillations with increasing pump level is easily determined by visual observation of the structure of the radiation field. You can also visually observe the effect of cavity adjustment on the structure of coherent radiation modes.
Gases are more homogeneous than condensed media. Therefore, the light beam in the gas is less distorted and scattered, and the radiation of a helium-neon laser is characterized by good frequency stability and high directivity, which reaches its limit due to diffraction phenomena. Diffraction limit of divergence for a confocal cavity
|
|
,where λ – wavelength; d 0 is the diameter of the light beam in its narrowest part.
The radiation of a helium-neon laser is characterized by a high degree of monochromaticity and coherence. The emission line width of such a laser is much narrower than the “natural” spectral line width and is many orders of magnitude less than the maximum resolution of modern spectrometers. Therefore, to determine it, the beat spectrum of various modes in the radiation is measured. In addition, the radiation of this laser is plane-polarized due to the use of windows located at the Brewster angle to the optical axis of the resonator.
Evidence of the coherence of radiation can be observed by observing the diffraction pattern when radiation received from different points of the source is superimposed. For example, coherence can be assessed by observing the interference from a system of multiple slits. From Young's experience it is known that to observe the interference of light from an ordinary “classical” source, the radiation is first passed through one slit, and then through two slits, and then interference fringes are formed on the screen. In the case of using laser radiation, the first slit is unnecessary. This circumstance is fundamental. In addition, the distance between two slits and their width can be disproportionately greater than in classical experiments. At the exit window of the gas laser there are two slits, the distance between which is 2 a. In the case when the incident radiation is coherent, on a screen located at a distance d from the slits, an interference pattern will be observed. In this case, the distance between the maxima (minimum) of the bands
|
|
.1) active substance; 2) a pumping source that brings the active substance into an excited state; 3) an optical resonator consisting of two mirrors parallel to each other (Fig. 20)
Rice. 20.
Helium-neon laser is a laser whose active medium is a mixture of helium and neon. Helium-neon lasers are often used in laboratory experiments and optics. It has a working wavelength of 632.8 nm, located in the red part of the visible spectrum.
Helium-neon laser device
The working fluid of a helium-neon laser is a mixture of helium and neon in a ratio of 5:1, located in a glass flask under low pressure (usually about 300 Pa). The pumping energy is supplied from two electric dischargers with a voltage of about 1000-5000 volts (depending on the length of the tube), located at the ends of the flask. The resonator of such a laser usually consists of two mirrors - a completely opaque one on one side of the bulb and a second one that transmits about 1% of the incident radiation on the output side of the device.
Helium-neon lasers are compact, the typical cavity size is from 15 cm to 2 m, and their output power varies from 1 to 100 mW.
Operating principle
Helium-neon laser. The glowing beam in the center is an electrical discharge.
In a gas discharge in a mixture of helium and neon, excited atoms of both elements are formed. It turns out that the energies of the metastable level of helium 1 S 0 and the radiative level of neon 2p 5 5s I turn out to be approximately equal - 20.616 and 20.661 eV, respectively. The transfer of excitation between these two states occurs in the following process:
He* + Ne + ДE He + Ne*
and its efficiency turns out to be very large (where (*) shows the excited state, and DE is the difference in the energy levels of two atoms.) The missing 0.05 eV is taken from the kinetic energy of motion of the atoms. The population of the neon level 2p 5 5s I increases and at a certain moment becomes greater than that of the underlying level 2p 5 3p I. An inversion of the level population occurs—the medium becomes capable of laser generation.
When a neon atom transitions from the 2p 5 5s І state to the 2p 5 3p І state, radiation with a wavelength of 632.816 nm is emitted. The 2p 5 3p I state of the neon atom is also radiative with a short lifetime and therefore this state is quickly deexcited into the 2p 5 3s level system and then into the ground state 2p 6 - either due to the emission of resonant radiation (radiating levels of the 2p 5 3s system) , or due to collision with walls (metastable levels of the 2p 5 3s system).
In addition, with the correct choice of cavity mirrors, it is possible to obtain laser lasing at other wavelengths: the same 2p 5 5s I level can transition to 2p 5 4p I with the emission of a photon with a wavelength of 3.39 μm, and the 2p 5 4s I level arising at collision with another metastable level of helium, can go to 2p 5 3p I, emitting a photon with a wavelength of 1.15 μm. It is also possible to obtain laser radiation at wavelengths of 543.5 nm (green), 594 nm (yellow) or 612 nm (orange).
The bandwidth in which the effect of amplification of radiation by the laser working body is preserved is quite narrow and amounts to about 1.5 GHz, which is explained by the presence of a Doppler shift. This property makes helium-neon lasers good radiation sources for use in holography, spectroscopy, and barcode reading devices.
Ruby laser
The laser consists of three main parts: an active (working) substance, a resonant system, which consists of two parallel plates with reflective coatings applied to them, and an excitation (pumping) system, which is usually a xenon flash lamp with a power source.
Ruby is an aluminum oxide in which some of the aluminum atoms are replaced by chromium atoms (Al2O3*Cr2O3). The active substance is chromium ions Cr 3+. Its color depends on the chromium content in the crystal. Typically a pale pink ruby is used, containing about 0.05% chromium. The ruby crystal is grown in special furnaces, then the resulting workpiece is annealed and processed, giving it the shape of a rod. The length of the rod ranges from 2 to 30 cm, the diameter from 0.5 to 2 cm. The flat end ends are made strictly parallel, ground and polished with high precision. Sometimes reflective surfaces are applied not to individual reflective plates, but directly to the ends of the ruby rod. The surfaces of the ends are silvered, and the surface of one end is made completely reflective, the other - partially reflective. Typically, the light transmittance of the second end is about 10-25%, but it can be different.
The ruby rod is placed in a spiral pulsed xenon lamp, the coils of which surround it on all sides. The lamp flash lasts milliseconds. During this time, the lamp consumes energy of several thousand joules, most of which is spent on heating the device. Another, smaller part, in the form of blue and green radiation, is absorbed by ruby. This energy provides the excitation of chromium ions.
In a normal, unexcited state, chromium ions are located at the lower level 1. When ruby is irradiated with light from a xenon lamp containing the green part of the spectrum, chromium atoms are excited and move to the upper level 3, corresponding to the absorption of light with a wavelength of 5600 A. The absorption band width of this level is about 800 A.
From level 3, some of the excited chromium atoms return to the main level 1, and some go to level 2. This is the so-called non-radiative transition, in which chromium ions give up part of their energy to the crystal lattice in the form of heat. The probability of moving from level 3 to level 2 is 200 times greater, and from level 2 to level 1 is 300 times less than from level 3 to level 1. This leads to the fact that level 2 is more populated than level 1. Others In other words, the population becomes inverted and the necessary conditions for intense induced transitions are created.
Such a system is extremely unstable. The probability of spontaneous transitions at any time is very high. The very first photon that appears during a spontaneous transition, according to the law of induced radiation, will knock out a second photon from a neighboring atom, transferring the emitting atom to the ground state. Then these two photons will knock out two more, after which there will be four of them, etc. The process increases almost instantly. The first wave of radiation, having reached the reflecting surface, will turn back and cause a further increase in the number of induced transitions and radiation intensity. Reflection from the reflecting surfaces of the resonator will be repeated many times, and if the power loss during reflection, caused by the imperfection of the reflective coatings, as well as the translucency of one of the ends of the rod, through which the radiation flux will burst out at the beginning of generation, will not exceed the power that is acquired as a result of the onset of generation of a beam formed in the laser rod, the generation will increase and the power will increase until the majority of the excited particles of the active substance (chromium ions) give up their energy acquired at the moment of excitation. A beam of very high intensity will be released through the partially silvered end of the rod. The direction of the beam will be strictly parallel to the axis of the ruby.
Those photons, the direction of propagation of which at the beginning of their occurrence did not coincide with the axis of the rod, will go through the side walls of the rod without causing any noticeable generation.
It is the repeated passage of the generated light wave between the end walls of the resonator without any significant deviation from the axis of the rod that provides the beam with strict directionality and enormous output power.
27. Helium-neon laser.
A laser whose active medium is a mixture of helium and neon. Helium-neon lasers are often used in laboratory experiments and optics. It has a working wavelength of 632.8 nm, located in the red part of the visible spectrum.
The working fluid of a helium-neon laser is a mixture of helium and neon in a ratio of 5:1, located in a glass flask under low pressure (usually about 300 Pa). The pumping energy is supplied from two electric dischargers with a voltage of about 1000 volts, located at the ends of the bulb. The resonator of such a laser usually consists of two mirrors - completely opaque on one side of the bulb and the second, transmitting about 1% of the incident radiation on the output side of the device. Helium-neon lasers are compact, the typical size of the resonator is from 15 cm to 0.5 m, their output power varies from 1 to 100 mW.
Operating principle: In a gas discharge in a mixture of helium and neon, excited atoms of both elements are formed. It turns out that the energies of the metastable level of helium 1S0 and the radiative level of neon 2p55s² are approximately equal - 20.616 and 20.661 eV, respectively. The transfer of excitation between these two states occurs in the following process: He* + Ne + ΔE → He + Ne* and its efficiency turns out to be very high (where (*) shows the excited state, and ΔE is the difference in the energy levels of the two atoms.) The missing 0.05 eV are taken from the kinetic energy of atomic motion. The population of the neon level 2p55s² increases and at a certain moment becomes larger than that of the underlying level 2p53p². An inversion of the level population occurs - the medium becomes capable of laser generation. When a neon atom transitions from the 2p55s² state to the 2p53p² state, radiation with a wavelength of 632.816 nm is emitted. The 2p53p state of the neon atom is also radiative with a short lifetime and therefore this state is quickly deexcited into the 2p53s level system and then into the 2p6 ground state - either due to the emission of resonant radiation (emitting levels of the 2p53s system), or due to collision with the walls ( metastable levels of the 2p53s system). In addition, with the correct choice of cavity mirrors, it is possible to obtain laser lasing at other wavelengths: the same 2p55s² level can go to 2p54p² with emission of a photon with a wavelength of 3.39 μm, and the 2p54s² level arising during a collision with a different metastable level of helium, can switch to 2p53p², emitting a photon with a wavelength of 1.15 μm. It is also possible to obtain laser radiation at wavelengths of 543.5 nm (green), 594 nm (yellow) or 612 nm (orange). The bandwidth in which the effect of amplification of radiation by the laser working body remains is quite narrow, and is about 1.5 GHz, which is explained by the presence of a Doppler shift. This property makes helium-neon lasers good radiation sources for use in holography, spectroscopy, and barcode reading devices.
The most common gas laser is helium-neon ( He-Ne) laser (neutral atom laser), which operates on a mixture of helium and neon in a ratio of 10:1. This laser is also the first continuous laser.
Let's consider the energy diagram of the helium and neon levels (Fig. 3.4). Generation occurs between neon levels, and helium is added to carry out the pumping process. As can be seen from the figure, the levels 2 3 S 1 And 2 1 S 0 helium are located, accordingly, close to the levels 2s And 3s not she. Because helium levels 2 3 S 1 And 2 1 S 0 are metastable, then when metastable excited helium atoms collide with neon atoms, a resonant energy transfer to the neon atoms will occur (collisions of the second kind).
So the levels 2s And 3s neon can be populated and, therefore, generation can occur from these levels. Lifetime s-states ( ts»100 ns) much longer lifetime R-states ( t r»10 ns), therefore the condition for the laser to operate according to a four-level scheme is met:
1 1 S Þ (3s, 2s) Þ(3p,2p) Þ 1s .
Laser generation is possible at one of the transitions a, b, c according to the wavelengths l a=3.39 µm, l b=0.633 µm, l with=1.15 µm, which can be obtained by selecting the reflectance of the resonator mirrors or by introducing dispersive elements into the resonator.
Rice. 3.4. Diagram of the energy levels of helium and neon.
Let us consider the lasing characteristics of such a laser.
Fig.3.5. Lasing characteristics of a helium-neon laser.
The initial increase in output power with increasing pump current is explained by population inversion. After reaching the maximum power, with a further increase in the pump current, the curve begins to decrease. This is explained by the fact that the 2p and 1s levels do not have time to relax, i.e. electrons do not have time to move to a low energy level and the number of electrons in neighboring 2p and 1s levels becomes the same. In this case there is no inversion.
The efficiency of helium-neon lasers is on the order of 0.1%, which is explained by the low volume density of excited particles. Output power typical He-Ne–laser P~5-50 mW, divergence q~1 mrad.
Argon laser
These are the most powerful continuous lasers in the visible and near ultraviolet region of the spectrum related to ion gas lasers. The upper laser level in the working gas is populated by two successive collisions of electrons during an electrical discharge. During the first collision, ions from neutral atoms are formed, and during the second, these ions are excited. Therefore, pumping is a two-step process, the efficiency of each step being proportional to the current density. Sufficiently high current densities are required for efficient pumping.
Laser energy level diagram on Ar+ shown in Fig. 3.3. Laser emission in the lines between 454.5 nm and 528.7 nm occurs when a group of levels is populated 4p by electron impact excitation of ground or metastable states Ar+.
3.5 CO 2 laser
Molecular CO 2– lasers are the most powerful continuous lasers among gas lasers, due to the highest efficiency of conversion of electrical energy into radiation energy (15-20%). Laser generation occurs at vibrational-rotational transitions and the emission lines of these lasers are in the far-IR region, which are located at wavelengths of 9.4 μm and 10.4 μm.
IN CO 2– the laser uses a mixture of gases CO 2, N 2 And He. Pumping is carried out directly during collisions of molecules CO 2 with electrons and vibrationally excited molecules N 2. High thermal conductivity of He in the mixture promotes cooling CO 2, which leads to depletion of the lower laser level, populated as a result of thermal excitation. So the presence N 2 in the mixture contributes to a high population of the upper laser level, and the presence He– depletion of the lower level, and ultimately together they lead to an increase in population inversion. Energy Level Diagram CO 2-laser is shown in Fig. 3.4. Laser generation occurs during a transition between the vibrational states of a molecule CO 2 n 3 Þn 1 or n 3 Þn 2 with a change in rotational state.
Rice. 3.4. Energy Level Diagram N 2 And CO 2 V CO 2–laser.
CO 2– the laser can operate in both continuous and pulsed modes. In continuous mode, its output power can reach several kilowatts.
The helium-neon laser, along with diode or semiconductor lasers, is one of the most commonly used and most affordable lasers for the visible region of the spectrum. The power of laser systems of this kind, intended mainly for commercial purposes, ranges from 1 mW to several tens of mW. Especially popular are not so powerful He-Ne lasers of the order of 1 mW, which are used mainly as quoting devices, as well as for solving other problems in the field of measurement technology. In the infrared and red ranges, the helium-neon laser is increasingly being replaced by the diode laser. He-Ne lasers are capable of emitting orange, yellow and green lines in addition to red lines, which is achieved thanks to appropriate selective mirrors.
Energy Level Diagram
The energy levels of helium and neon that are most important for the function of He-Ne lasers are shown in Fig. 1. Laser transitions occur in the neon atom, with the most intense lines resulting from transitions with wavelengths 633, 1153 and 3391 (see Table 1).
The electronic configuration of neon in the ground state looks like this: 1s22s22p6, with the first shell (n = 1) and the second shell (n = 2) filled with two and eight electrons, respectively. Higher states in Fig. 1 arise as a result of the fact that there is a 1s22s22p5 shell, and the luminous (optical) electron is excited according to the scheme: 3s, 4s, 5s,..., Зр, 4р,... etc. We are therefore talking about a one-electron state that communicates with the shell. In the LS (Russell - Saunders) scheme, the energy levels of neon are given a single-electron state (for example, 5s), as well as the resulting total orbital momentum L (= S, P, D...). In the notation S, P, D,..., the lower index shows the total orbital momentum J, and the upper index indicates the multiplicity 2S + 1, for example, 5s1P1. Often, a purely phenomenological designation according to Paschen is used (Fig. 1). In this case, the sublevels of excited electronic states are counted from 2 to 5 (for s-states) and from 1 to 10 (for p-states).
Rice. 1. Diagram of energy levels of a He-Ne laser. For neon, the levels are designated according to Paschen, that is: 3s2, 3s3, 3s4, 3s5, etc.
Table 1. Designations of transitions of intense lines of the He-Ne laser
Excitation
The active medium of a helium-neon laser is a gas mixture to which the necessary energy is supplied in an electrical discharge. The upper laser levels (2s and 2p according to Paschen) are selectively populated based on collisions with metastable helium atoms (23S1, 21S0). During these collisions, not only kinetic energy is exchanged, but also the energy of excited helium atoms is transferred to neon atoms. This process is called a collision of the second kind:
He* + Ne -> He + Ne* + ΔE, (1)
where the asterisk (*) symbolizes the excited state. The energy difference in the case of excitation of the 2s level is: &DeltaE=0.05 eV. During a collision, the existing difference is converted into kinetic energy, which is then distributed as heat. For the 3s level, identical relationships hold. This resonant energy transfer from helium to neon is the main pumping process when creating a population inversion. In this case, the long lifetime of the metastable state does not have a favorable effect on the selectivity of population of the upper laser level.
The excitation of He atoms occurs based on the collision of electrons - either directly or through additional cascade transitions from higher levels. Due to long-lived metastable states, the density of helium atoms in these states is very high. The upper laser levels 2s and 3s can - taking into account the selection rules for electrical Doppler transitions - go only to the underlying p-levels. For successful generation of laser radiation, it is extremely important that the lifetime of s-states (upper laser level) = approximately 100 ns exceeds the lifetime of p-states (lower laser level) = 10 ns.
Wavelengths
Next, we will consider the most important laser transitions in more detail using Fig. 1 and data from table 1. The most famous line in the red region of the spectrum (0.63 μm) arises due to the transition 3s2 → 2р4. The lower level is split as a result of spontaneous emission within 10 ns into the 1s level (Fig. 1). The latter is resistant to splitting due to electric dipole radiation, so it is characterized by a long natural life. Therefore, atoms are concentrated in a given state, which turns out to be highly populated. In a gas discharge, atoms in this state collide with electrons, and then the 2p and 3s levels are excited again. At the same time, population inversion decreases, which limits the laser power. The depletion of the ls state occurs in helium-neon lasers mainly due to collisions with the wall of the gas-discharge tube, and therefore, as the diameter of the tube increases, a decrease in gain and a decrease in efficiency are observed. Therefore, in practice, the diameter is limited to approximately 1 mm, which, in turn, limits the output power of He-Ne lasers to several tens of mW.
The electronic configurations 2s, 3s, 2p and 3p participating in the laser transition are split into numerous sublevels. This leads, for example, to further transitions in the visible region of the spectrum, as can be seen from Table 2. For all visible lines of a He-Ne laser, the quantum efficiency is about 10%, which is not so much. The level diagram (Fig. 1) shows that the upper laser levels are located approximately 20 eV above the ground state. The energy of red laser radiation is only 2 eV.
Table 2. Wavelengths λ, output powers and linewidths Δ ƒ He-Ne laser (Paschen transition designations)
| Color | λ nm |
Transition (according to Paschen) |
Power mW |
Δ ƒ MHz |
Gain %/m |
| Infrared | 3 391 | 3s2 → 3p4 | > 10 | 280 | 10 000 |
| Infrared | 1 523 | 2s2 → 2p1 | 1 | 625 | |
| Infrared | 1 153 | 2s2 → 2p4 | 1 | 825 | |
| Red | 640 | 3s2 → 2p2 | |||
| Red | 635 | 3s2 → 2p3 | |||
| Red | 633 | 3s2 → 2p4 | > 10 | 1500 | 10 |
| Red | 629 | 3s2 → 2p5 | |||
| Orange | 612 | 3s2 → 2p6 | 1 | 1 550 | 1.7 |
| Orange | 604 | 3s2 → 2p7 | |||
| Yellow | 594 | 3s2 → 2p8 | 1 | 1 600 | 0.5 |
| Yellow | 543 | 3s2 → 2p10 | 1 | 1 750 | 0.5 |
Emission in the infrared range around 1.157 μm occurs through 2s → 2p transitions. The same applies to the slightly weaker line at approximately 1.512 µm. Both of these infrared lines are used in commercial lasers.
A characteristic feature of the line in the IR range at 3.391 μm is its high gain. In the area of weak signals, that is, with a single passage of weak light signals, it is about 20 dB/m. This corresponds to a factor of 100 for a laser 1 meter long. The upper laser level is the same as for the known red transition (0.63 μm). The high gain, on the one hand, is caused by the extremely short lifetime at the lower 3p level. On the other hand, this is explained by the relatively long wavelength and, accordingly, low frequency of radiation. Typically, the ratio of stimulated to spontaneous emissions increases for low frequencies ƒ. The amplification of weak signals g is usually proportional to g ~ƒ2.
Without selective elements, the helium-neon laser would emit at the 3.39 µm line rather than in the red region at 0.63 µm. The excitation of the infrared line is prevented either by the selective mirror of the resonator or by absorption in the Brewster windows of the gas-discharge tube. Thanks to this, the lasing threshold of the laser can be raised to a level sufficient to emit 3.39 µm, so that only a weaker red line appears here.
Design
The electrons necessary for excitation are generated in a gas discharge (Fig. 2), which can be used with a voltage of about 12 kV at currents from 5 to 10 mA. The typical discharge length is 10 cm or more, the diameter of the discharge capillaries is about 1 mm and corresponds to the diameter of the emitted laser beam. As the diameter of the gas-discharge tube increases, the efficiency decreases, since collisions with the tube wall are required to empty the ls-level. For optimal power output, the total filling pressure (p) is used: p·D = 500 Pa·mm, where D is the tube diameter. The He/Ne mixture ratio depends on the desired laser line. For the known red line we have He: Ne = 5:l, and for the infrared line about 1.15 μm - He:Ne = 10:l. Optimization of current density also seems to be an important aspect. The efficiency for the 633 nm line is about 0.1%, since the excitation process in this case is not very efficient. The service life of a helium-neon laser is about 20,000 operating hours.
Rice. 2. Design of a He-Ne laser for polarized radiation in the mW range
The gain under such conditions is at g=0.1 m-1, so it is necessary to use mirrors with high reflectivity. To exit the laser beam only on one side, a partially transmitting (translucent) mirror is installed there (for example, with R = 98%), and on the other side - a mirror with the highest reflectivity (~ 100%). The gain for other visible transitions is much smaller (see Table 2). For commercial purposes, these lines have only been achieved in recent years using mirrors characterized by extremely low losses.
Previously, with a helium-neon laser, the output windows of the gas-discharge tube were fixed with epoxy resin, and the mirrors were mounted externally. This caused helium to diffuse through the glue and water vapor to enter the laser. Today, these windows are fixed by direct welding of metal to glass, which reduces helium leakage to approximately 1 Pa per year. In the case of small mass-produced lasers, the mirror coating is applied directly to the output windows, which greatly simplifies the entire design.
Beam properties
To select the direction of polarization, the gas-discharge lamp is equipped with two inclined windows or, as shown in Fig. 2, a Brewster plate is inserted into the resonator. The reflectivity on an optical surface becomes zero if the light is incident at the so-called Brewster angle and is polarized parallel to the plane of incidence. Thus, radiation with this direction of polarization passes through the Brewster window without loss. At the same time, the reflectivity of the component polarized perpendicular to the plane of incidence is quite high and is suppressed in the laser.
The polarization ratio (the ratio of power in the direction of polarization to the power perpendicular to this direction) is 1000:1 for conventional commercial systems. When a laser operates without Brewster plates with internal mirrors, unpolarized radiation is generated.
The laser usually generates in the transverse TEM00 mode (low-order mode), and several longitudinal (axial) modes are formed at once. When the distance between the mirrors (laser cavity length) is L = 30 cm, the intermode frequency interval is Δ ƒ` = c/2L = 500 MHz. The central frequency is at the level of 4.7·1014 Hz. Since light amplification can occur within the range Δƒ = 1500 MHz (Doppler width), at L = 30CM three different frequencies are emitted: Δƒ/Δƒ`= 3. When using a smaller mirror spacing (<= 10см) может быть получена одночастотная генерация. При короткой длине мощность будет весьма незначительной. Если требуется одночастотная генерация и более высокая мощность, можно использовать лазер большей длины и с оснащением частотно-селективными элементами.
Helium-neon lasers around 10 mW are often used in interferometry or holography. The coherence length of such mass-produced lasers ranges from 20 to 30 cm, which is quite sufficient for holography of small objects. Longer coherence lengths are obtained by using serial frequency-selective elements.
When the optical distance between the mirrors changes as a result of thermal or other effects, the axial natural frequencies of the laser cavity shift. With single-frequency generation, a stable radiation frequency is not obtained here - it moves uncontrollably in the line width range of 1500 MHz. By means of additional electronic regulation, frequency stabilization can be achieved precisely in the center of the line (for commercial systems, frequency stability of several MHz is possible). In research laboratories it is sometimes possible to stabilize a helium-neon laser to a range of less than 1 Hz.
By using suitable mirrors, different lines from Table 4.2 can be excited to generate laser radiation. The most commonly used visible line is around 633 nm with typical powers of several milliwatts. After suppression of an intense laser line around 633 nm, other lines in the visible range may appear in the cavity through the use of selective mirrors or prisms (see Table 2). However, the output power of these lines is only 10% of the output power of an intensive line or even less.
Commercial helium-neon lasers are available in a variety of wavelengths. In addition to them, there are also lasers that generate on many lines and are capable of emitting waves of many lengths in a variety of combinations. In the case of tunable He-Ne lasers, it is proposed to select the required wavelength by rotating the prism.
