The He-Ke laser is, without a doubt, the most significant among the | all inert gas lasers. Generation here is carried out by transitions of the neon atom, and helium is added to the gas mixture to increase the pumping efficiency. Does this laser emit at many wavelengths? waves, of which the best known is the line with k = 633 nm (red). The middle of other lines is green at a wavelength k = 543 nm and two lines in the IR range with k = 1.15 and 3.39 μm. A helium-non-ion laser, generating at a transition with a wavelength k = 1.15 μm, was the very first gas laser; moreover, continuous laser generation was demonstrated for the first time. g1
Fig. Figure 10.1 shows a simplified diagram of the energy levels of the He and Ke atoms. Levels are not indicated according to approximation! We use the Russell-Sanders connection, where the first digit indicates the principal quantum number of a given level. Thus, the 1x5 state is responsible?*| This corresponds to the case when both electrons of the He atom are in the 1* state with oppositely directed spins. The 235 and 2^ states correspond to the situation when one of the two electrons is thrown into the 2^ state and its spin is, respectively, parallel or antiparallel to the spin of the other electron. On the other hand:
Neon has an atomic number of 10 and uses a number of ways to designate energy levels, such as Paschen or Cancer notations. However, for simplicity, we will limit ourselves to only designating the electronic configuration for each corresponding level. Thus, the ground state of neon is denoted as 1822822p6, while the excited states shown in the figure correspond to the situation when one 2p electron is thrown into an excited 8- (38-, 48-OR 5v) OR excited - (3P" or 4p) state It should also be noted that due to the interaction with the five electrons remaining in the 2p orbitals, these 8- and p-states are split into 4 and 10 sublevels, respectively.
From Fig. 10.1 it is obvious that in the He atom the 23В and 2*В levels are close to resonance with the 4$ and 5В states of the N6 atom. Since the levels 2Sv and 2*v are metastable (transitions in -> in are prohibited in the electric dipole approximation; and, moreover, the transition 23v -> 2xv is also prohibited from the point of view of changes in multiplicity, i.e., in spin), the atoms It is not in these states that they turn out to be a very effective means for excitation of the 4b and 58 levels of Lie atoms (via resonant energy transfer). It was found that in a He-Ke laser it is precisely this excitation mechanism that is dominant in obtaining population inversion, although pumping, in addition, can also be carried out due to collisions of electrons with Ge atoms. Since the 4b and 6b levels of the Eu atom can be quite heavily populated, they are well suited for the role of upper levels of laser transitions. Taking into account the selection rules, it can be seen that the possible transitions here are transitions to p-states. Moreover, it should be noted that the relaxation time of 8-states (t8 = 100 ns) is an order of magnitude greater than the relaxation time of p-states (tr = 10 ns), thus, the condition of continuous generation (7.3.1) is satisfied. Finally, it should be noted that the probability of excitation from the ground state to levels 3p and 4p (due to electron impact), due to smaller interaction cross sections, turns out to be significantly lower than the corresponding probabilities of excitation to levels 4" and 58. Nevertheless, as will be seen Below, direct excitation to levels 3p and 4p also has a significant effect on laser operation.
From the above it follows that lasing in neon can be expected between 58 or 48 levels (playing the role of upper laser levels) and 3p or 4p levels, which can be considered as lower laser levels. In Fig. Figure 10.1 shows some of the most important laser transitions that occur between these states. For transitions with very different wavelengths (ξk > 0.2A), each specific transition at which generation will occur is determined by the wavelength to which the maximum reflection coefficient of the multilayer dielectric mirror is “tuned” (see Fig. 4.9). Laser transitions are broadened mainly due to the Doppler effect. For example, for the red He-Me transition (X = 633 nm in vacuum and X = 632.8 nm in air), Doppler broadening leads to the fact that the width of this line is of the order of ~1.5 GHz (see also example 2.6). For comparison, from expression (2.5.13) we can estimate the magnitude of the intrinsic broadening: Ауа1 = 1/(2пх) = 19 MHz, where
|
Spectroscopic properties of laser transitions, as well as the composition of the gas mixture in some of the most common atomic and ion gas lasers
|
T-1 = t’1 + Tp1, a and tp are the lifetimes of 8-ir states, respectively. The broadening associated with collisional processes turns out to be even less than its own broadening (for example, for pure Ke we have Duc = 0.6 M1^ at a pressure p = 0.5 mm Hg; see example 2.2). Some spectroscopic properties of the laser transition corresponding to the wavelength 633 ted are given in Table. 10.1.
In Fig. Figure 10.2 shows the basic design of a He-N laser. The discharge emanates between the ring-shaped anode and a large cathode shaped like a tube. In this case, positive ions collide with this cathode. For most of the length of the tube, the discharge is formed in the capillary, and only in this region is a high population inversion achieved. The large total volume of gas surrounding the capillary plays the role of a reservoir for the replenishment of the He-N mixture in the capillary. In the case when it is necessary to obtain polarized radiation as output from a laser, a plate is placed inside the tube at a Brewster angle. The laser mirrors are directly soldered into the ends of the tube. The resonator configuration close to the floor is most often used<
ical, since it is easily adjusted, is very stable in terms of misalignment and easily provides lasing on the TEM00 mode. The only drawback of this configuration is that it does not fully utilize the volume of the plasma discharge, since the size of the mode spot on a flat mirror is much smaller than on a concave one. However, if in Fig. 10.2, the flat mirror is placed on the left, then the region with a smaller spot size for the almost hemispherical TEM00 mode will be outside the capillary, i.e., in the region of low inversion.
One of the most characteristic features of the He-Ke laser is that its output power does not increase monotonically with increasing discharge current, but reaches a maximum and then decreases. Therefore, commercially produced He-Ke lasers are provided with a power source designed only for optimal current. The presence of an optimal current value, i.e., current density J flowing through the capillary, is due (at least for transitions 0.633 and 3.39 μm) to the fact that at high current densities, the deactivation of metastable states (23В and 21Г) of the He atom occurs not only due to collisions with walls, but also during superelastic collisions, for example:
He(215) + e -> He(11c) + e. (10.2.1)
Since the rate of this process is proportional to the electron density E, and therefore J, the total deactivation rate can be written as k2 + **7. In this expression, k2 is a constant characterizing deactivation due to collisions with walls, and k&1 (where &3 is also a constant number) represents the rate of processes associated with superelastic collisions (10.2.1). On the other hand, the excitation speed can be written as &1C/, where kx is again a constant. Under stationary conditions, we can write = (k2 + k#1)I*, where - nas
The density of the ground state of the He atom, and LG* is the population of the excited state 215. The equilibrium value of the population of the 2X£ level is given by the expression:
Къ+кГ (10.2.2)
From which it can be seen that at high current densities, population saturation occurs. Since the equilibrium population of the 6b state of the N6 atom is determined by the near-resonant energy transfer from the 2^ state, the population of the upper laser level 5b will also be saturated with increasing current density *1 (Fig. 10.3). On the other hand, it was experimentally discovered that in the absence of lasing, the population of the lower laser level (3p or 4p) continues to grow linearly with increasing J (Fig. 10.3) due to the direct pumping of Ge atoms from the ground state and cascade radiative transitions from the upper laser levels .
Thus, as the discharge current density increases, the population difference, and with it the output power, increases to a certain optimal value and then decreases.
In addition to the indicated optimal value of current density, the He-Ne laser also has other optimal operating parameters. In particular, these include:
■ the optimal value of the product of the total gas pressure p and the tube diameter B (p!) = 3.6 - 4 mm Hg. Art. * mm). The existence of an optimal pB value indicates the presence of some optimal electron temperature (see Section 6.4.5);
■ optimal ratio of gas partial pressure He to gas pressure Ge (~5:1 for wavelength X = 632.8 nm and -9:1 for X = 1.15 μm);
■ the optimal value of the capillary diameter (P = 2 mm). This can be explained
The thread is as follows: at a constant value of p£>, i.e. at a constant electron temperature, the number of all excitation processes (due to electron impact) is simply reduced to the number of atoms that can be excited; and since both the upper and lower laser holes are not populated, ultimately, by the electron impact, their populations, and therefore the laser gain, are directly proportional to the pressure p, or the value of I) -1, with a constant product p £> . On the other hand, the diffraction losses of the laser cavity will increase as the parameter I decreases, and thus one can obtain: optimal capillary diameter by optimizing the net gain (gain minus diffraction loss).
According to the dependence shown in Fig. 10.3, power of the Un-seen hole*|
The level is usually small (when optimizing laser parameters, the output power at wavelength X = 633 nm is in the range of 1-10 mW for tube lengths from 20 to 50 cm, while the output power at the green transition is usually an order of magnitude less). Efficiency He -Y laser on all laser transitions turns out to be very low (< 10_3). Главной причиной столь низкого КПД является мала# величина квантовой эффективности лазера. Действительно, из рис. 10.1 вид - ; но, что каждый элементарный процесс накачки требует затраты энергии около 20 эВ, в то время как энергия лазерного фотона не превышает 2 эВ.)
On the other hand, the presence of a very narrow gain line in such a laser is an obvious advantage when obtaining lasing in a single mode. Indeed, if the length of the resonator is small enough! (b< 15-20 см), генерацию на одной продольной моде можно с легкостью реа* лизовать путем тонкой подстройки длины резонатора (например, с помощью пьезокерамического устройства), добиваясь, таким образом, совпадения частоты моды с центром контура усиления (см. раздел 7.8.2.1). В одномодовом Не-Ке лазере можно обеспечить очень high degree stabilization of the frequency [Dn/y = 10"11 - g-1012] according to the Lamb dip using a reference frequency (for example, a Fabry-Perot interferometer with a large sharpness value), and an even better degree of stabilization can be achieved by using a reversed Lamb dip using an absorbing cell containing element 12912 (for the transition at a wavelength of 633 nm).
He-N lasers generating at the red transition are still widely used in many areas where low-power coherent radiation in the visible range is required (for example, for adjusting devices or reading bar codes). Most supermarkets and other retail outlets use red Hex lasers to read the information contained in the barcode of each product. However, here the main competition for He-Ke lasers comes from semiconductor lasers emitting in the red range, which turn out to be more compact and much more efficient. However, non-green lasers, due to the fact that green light is much better perceived by the eye, are all to a greater extent used in instrument adjustment, as well as in cell cytometry. In the latter case, the following occurs: separated cells (for example, red blood cells), stained with suitable fluorochromes, quickly flow through a capillary on which the He-N laser beam is focused, after which the stained cells can be detected by the corresponding scattering or fluorescence signals. In addition, single-frequency He-N lasers are often used in metrology applications (for example, in very precise interference distance measuring devices) and in holography.
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 its ground state looks like this: 1 s 2 2s 2 2p 6 and the first shell ( n= 1) and the second shell ( n= 2) are filled with two and eight electrons, respectively. Higher states in Fig. 1 arise as a result of the fact that there is 1 s 2 2s 2 2p 5-shell, and the luminous (optical) electron is excited according to the scheme: 3 s, 4s, 5s,..., Z R, 4R,... etc. We are therefore talking about a one-electron state that communicates with the shell. In the LS (Russell - Saunders) scheme, a single-electron state is indicated for the energy levels of neon (for example, 5 s), 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, 5 s 1 P 1 . 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).
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 (2 3 S 1, 2 1 S 0). 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. In the event of a collision, the existing difference is converted to 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. To successfully generate 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 3s 2 → 2p 4. 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, the atoms are concentrated in this 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 He-Ne laser quantum efficiency is about 10%, which is not that 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 | 3s 2 → 3p 4 | > 10 | 280 | 10 000 |
| Infrared | 1 523 | 2s 2 → 2p 1 | 1 | 625 | |
| Infrared | 1 153 | 2s 2 → 2p 4 | 1 | 825 | |
| Red | 640 | 3s 2 → 2p 2 | |||
| Red | 635 | 3s 2 → 2p 3 | |||
| Red | 633 | 3s 2 → 2p 4 | > 10 | 1500 | 10 |
| Red | 629 | 3s 2 → 2p 5 | |||
| Orange | 612 | 3s 2 → 2p 6 | 1 | 1 550 | 1.7 |
| Orange | 604 | 3s 2 → 2p 7 | |||
| Yellow | 594 | 3s 2 → 2p 8 | 1 | 1 600 | 0.5 |
| Yellow | 543 | 3s 2 → 2p 10 | 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.
Characteristic feature line in the IR range at 3.391 µm is 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. Usually the ratio of forced and spontaneous emissions increases for low frequencies ƒ. The amplification of weak signals g is generally proportional to g ~ƒ 2 .
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 the level of 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 could only be obtained in last 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 TEM 00 mode (lowest 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 4.7·10 14 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.
Features of the gaseous active medium. Basic methods of excitation. Electric discharge, gas dynamics, chemical excitation, photodissociation, optical pumping. Resonant transfer of excitation energy during collisions. Helium-neon laser. Level diagram. Transfer of excitation energy. Competition between emission lines at 3.39 and 0.63 µm. Discharge parameters, laser parameters.
We will consider methods for creating inversion using examples of lasers that are of the greatest interest.
Let's start with gas lasers. The gaseous nature of their active medium leads to a number of remarkable consequences. First of all, only gaseous media can be transparent in a wide spectral range from the vacuum UV region of the spectrum to waves in the far IR, essentially microwave, range. As a result, gas lasers operate over a huge range of wavelengths, corresponding to a change in frequency of more than three orders of magnitude.
Further. Compared to solids and liquids, gases have a significantly lower density and higher homogeneity. Therefore, the light beam in the gas is less distorted and scattered. This makes it easier to reach the diffraction limit of laser radiation divergence.
At low densities, gases are characterized by Doppler broadening of spectral lines, the magnitude of which is small compared to the width of the luminescence line in condensed matter. This makes it easier to achieve high monochromatic radiation from gas lasers. As a result, the characteristic properties of laser radiation - high monochromaticity and directionality - are most clearly manifested in the radiation of gas lasers.
The constituent particles of a gas interact with each other in the process of gas-kinetic collisions. This interaction is relatively weak; therefore, it practically does not affect the location of the particle energy levels and is expressed only in the broadening of the corresponding spectral lines. At low pressures, the collisional broadening is small and does not exceed the Doppler broadening
width. At the same time, an increase in pressure leads to an increase in the collision width (see lecture two), and we get the opportunity to control the width of the gain line of the active medium of the laser, which exists only in the case of gas lasers.
As we know, to satisfy the self-excitation conditions, the gain in the active medium during one pass of the laser cavity must exceed the losses. In gases, the absence of non-resonant energy losses directly in the active medium facilitates the fulfillment of this condition. It is technically difficult to produce mirrors with losses noticeably less than 1%. Therefore, the gain per pass must exceed 1%. The relative ease of meeting this requirement in gases, for example by increasing the length of the active medium, explains the availability of a large number of gas lasers in a wide range of wavelengths. At the same time, the low density of gases prevents the production of such a high density of excited particles, which is characteristic of solids. Therefore, the specific energy output of gas lasers is significantly lower than that of condensed matter lasers.
The specificity of gases is also manifested in the variety of different physical processes used to create population inversion. These include excitation during collisions in an electric discharge, excitation in gas-dynamic processes, chemical excitation, photodissociation, optical pumping (mainly by laser radiation), and electron-beam excitation.
In the vast majority of gas lasers, population inversion is created in an electrical discharge. Such gas lasers are called gas-discharge lasers. The gas-discharge method of creating an active medium is the most common method for obtaining inversion in gas lasers, since discharge electrons easily excite gas particles, transferring them to higher energy levels in the processes of inelastic collisions. The usually observed glow of a gas discharge (gas-light lamps) is explained by spontaneous transitions from these energy levels down. If the rates of decay processes of excited states are favorable to the accumulation of particles at some upper energy level and the depletion of some lower energy level, then a population inversion is created between these levels. By easily exciting the gas in a wide energy range, gas discharge electrons create an inversion of the populations of the energy levels of neutral atoms, molecules, and ions.
The gas-discharge method is applicable to excite lasers in both continuous and pulsed operating modes. Pulsed excitation is used mostly in the case of population dynamics at the upper and lower energy levels that are unfavorable for the continuous mode, as well as in order to obtain high radiation power that is unattainable in the continuous mode.
An electric discharge in a gas can be self-sustaining or non-self-sustaining. In the latter case, gas conductivity is ensured by an external ionizing agent, and the excitation process is carried out regardless of the gas breakdown conditions at the optimal value of the electric field strength in the discharge gap. In a gaseous medium ionized independently by an external influence, this field and the current caused by it determine the excitation energy (energy input) introduced into the discharge.
A characteristic feature of gases is the possibility of creating such flows of gas masses in which the thermodynamic parameters of the gas change sharply. Thus, if a preheated gas suddenly expands, for example, when flowing at supersonic speed through a nozzle, then the temperature of the gas drops sharply. This new, significantly lower temperature corresponds to a new equilibrium distribution of populations over the energy levels of gas particles. With a sudden decrease in gas temperature, the equilibrium of this distribution is disrupted for some time. Then, if relaxation to a new thermodynamic equilibrium for the lower level proceeds faster than for the upper level, the gasdynamic outflow is accompanied by a population inversion that exists in some extended region downstream of the gas. The size of this region is determined by the speed of the gas-dynamic flow and the relaxation time of the inverse population in it.
This is the gas-dynamic method of obtaining inversion, in which the thermal energy of a heated gas is directly converted into the energy of monochromatic electromagnetic radiation. An important characteristic feature of this method is the possibility of organizing gas-dynamic flows of large masses of the active substance and thereby obtaining high output power (see formula (6.57)).
During chemical excitation, population inversion is created as a result of chemical reactions in which excited atoms, molecules, and radicals are formed. The gas environment is convenient for chemical excitation because the reagents are easily and quickly mixed and easily transported. In gas-phase chemical reactions, the nonequilibrium distribution of chemical energy among the reaction products is most pronounced and persists for the longest time. Chemical lasers are interesting because they directly convert chemical energy into the energy of electromagnetic radiation. The involvement of chain reactions leads to a decrease in the relative share of energy consumption. expenses for initiating reactions that provide inversion. As a result, electricity consumption during operation of a chemical laser can be very small, which is also a great advantage of the chemical method of creating inversion. Let us add to this that the removal of reaction products, i.e., operation in a gas flow, can provide a continuous
operation of chemical lasers. A combination of chemical and gas-dynamic excitation methods is also possible.
Chemical lasers are closely related to lasers in which population inversion is achieved using photodissociation reactions. As a rule, these are fast reactions initiated by an intense pulsed flash of light or explosion. As a result of dissociation, excited atoms or radicals arise. The explosive nature of the reaction determines the pulsed operating mode of such lasers. Due to the fact that, with appropriate initiation, photodissociation can simultaneously cover a large volume of the source gas, the pulse power and radiation energy during the photodissociation method of creating inversion can reach significant values.
In the case of gaseous active media, such a general method of creating inversion as optical pumping acquires a peculiar character. Due to the low density of gases, their resonance absorption lines are narrow. Therefore, optical pumping can be effective if the pump source is sufficiently monochromatic. Laser sources are usually used. The specificity of gases in the case of optical pumping is also manifested in the fact that, due to their low density, the depth of penetration of pump radiation into the gas can be large and the heat release when absorbing radiation can be small. As a rule, resonant optical pumping of gaseous media practically does not lead to a violation of their optical homogeneity.
When electron beam excitation of gaseous media occurs, the gas is ionized by high energy electrons (0.3-3 MeV). In this case, the energy of fast electrons of the primary beam, the total number of which is relatively small, is cascaded into the energy of a large number of slow electrons. The upper laser levels are excited by these low energy electrons (from a few to tens of electron volts). Since the path length of high-energy electrons in gases is quite large, the electron-beam excitation method is very convenient for creating an active medium of large volumes at high gas pressures, and gases of any composition.
Electron beam excitation is a flexible and at the same time powerful method that is practically always applicable. The great advantage of this method is also the possibility of combining it with other methods of creating the active medium of gas lasers
Before moving on to a specific consideration of how all these methods of creating inversion are implemented in certain gas laser systems of greatest interest, it is advisable to note two general circumstances.
Firstly, achieving inversion in a gaseous medium is greatly facilitated by the relative slowness of relaxation processes
in gases. As a rule, the corresponding rate constants are well known or can be studied experimentally relatively easily. In the short-wavelength region and for well-resolved transitions, the process that prevents the achievement and retention of inversion is the spontaneous decay of the upper level (see lecture two). The radiative lifetimes of atoms, molecules, and ions are also either well known or can be relatively well known. The values of these times, known for free particles, are valid for gases.
Secondly, gases are characterized by the transfer of excitation energy from particles of one type to particles of another type during inelastic collisions between them. Such transfer is more effective the more closely the energy levels of colliding particles match. The fact is that the always existing difference in the energy values of those states whose populations are exchanged during a collision leads to the fact that the transfer of excitation is accompanied by the release (or absorption) of kinetic energy
Here N is the density of excitation energy donor particles, n is the density of acceptors, and the asterisk denotes the excitation of the corresponding particle. The symbol K above the arrows in equation (13.1) denotes the rate constant of this reaction. Kinetic energy can be obtained from a reservoir of thermal energy of translational motion of gas particles (or transferred to this reservoir). In order for such a process to be effective, the energy transferred to the reservoir (received from the reservoir) in one collision should not exceed the average energy of thermal motion of one particle. In other words, the energy deficit of the states under consideration should be small:
In this case, the so-called resonant (quasi-resonant) transfer of excitation energy occurs.
In general terms, the process of energy transfer (13.1) is described by a rate equation of the form
where m is some effective relaxation time, and the rate constant for excitation energy transfer, as usual,
Here v is the speed of colliding particles, and the cross section of the transfer process o approaches the gas-kinetic cross section when condition (13.2) is met. On the right side of the equation
(13.3) the inverse process is taken into account. Assuming that the law of conservation of the number of particles is satisfied:
from (13.3) it is easy to obtain that under stationary conditions
Given that
the level of excitation of acceptors is achieved, which is the maximum possible for a given level of excitation of donors.
So, the process of collisional transfer of excitation energy from particles of one type to particles of another type, characteristic of gaseous media, is effective when condition (13.2) is met. This process is effective in creating an n-particle laser active medium by exciting the N-particles when condition (13.7) is satisfied.
Rice. 13.1. Transfer of excitation energy according to the scheme: straight arrow up - excitation of particles N, straight arrow down - emission by particles, wavy arrow down - relaxation of the lower laser level of particles n. The absence of intrinsic relaxation of particles is shown
The transfer of excitation energy significantly expands the possibilities of creating gas lasers, making it possible to separate the functions of storing excitation energy and subsequent radiation at the desired wavelength in the active medium. The process occurs in two stages. First, in one way or another, particles of an auxiliary gas are excited - a carrier of excess energy and acting as a donor of excitation energy. Then, in the processes of elastic collisions, energy is transferred from the carrier gas to particles of the working gas - the acceptor of excitation energy, thus populating their upper laser level. Upper; The energy level of the auxiliary gas must have a long intrinsic lifetime in order to store energy well. The process under consideration is shown schematically in Fig. 13.1.
The method under consideration has found wide application, since with almost all excitation methods (electric discharge,
gasdynamic, chemical, etc.) it often turns out to be much more profitable to directly invest excitation energy not in those particles whose radiation is desired, but in those that easily absorb this energy, do not emit it themselves and willingly give up their excitation to the desired particles.
Let us now move on to a direct examination of a number of gas lasers. Let's start with atomic gas systems, a prominent example of which is the helium-neon laser. It is well known that this laser was, in essence, the first. The original calculations and proposals related to gas lasers, mainly due to the greater degree of understanding we have already discussed of energy level patterns and excitation conditions in a gas environment. Nevertheless, the ruby laser was the first to be created due to the fact that this single crystal was carefully studied in EPR radio spectroscopy and was widely used in microwave quantum electronics to create paramagnetic quantum amplifiers (paramagnetic masers). Soon, at the end of the same 1960, A. Javan,
Rice. 13.2. Scheme of excitation of neon and helium in an electric discharge (arrow symbols are the same as in Fig. 13.1). The possibility of cascade population of neon energy levels is demonstrated.
W. Bennett and D. Harriot created a helium-neon laser at a wavelength of 1.15 microns. The greatest interest in gas lasers arose after the discovery of generation of a helium-neon laser at the red line of 632.8 nm under almost the same conditions as in the first launch at a wavelength of 1.15 microns. This primarily stimulated interest in laser applications. The laser beam has become a tool.
Technical improvements have led to the fact that the helium-neon laser has ceased to be a miracle of laboratory technology and experimental art and has become a reliable device. This laser is well known, it lives up to its fame and deserves attention.
In a helium-neon laser, the working substance is neutral neon atoms. Excitation is carried out by electrical discharge. A simplified and at the same time, in a sense, generalized diagram of neon levels is shown on the right side of Fig. 13.2. In an electrical discharge during collisions with electrons
levels are excited. Levels are metastable, and level is shorter-lived in comparison. Therefore, it would seem that an inversion of level populations should easily occur with respect to . This, however, is prevented by the metastable level. In the spectra of many atoms, including atoms of inert gases, there is such a long-lived metastable level. By being populated in collisions with an electron, this level does not allow the level to become empty, which prevents the inversion from occurring.
It is difficult to create an inversion in continuous mode in pure neon. This difficulty, which is quite general in many cases, is overcome by introducing an additional gas into the discharge - a donor of excitation energy. This gas is helium. The energies of the first two excited metastable levels of helium (Fig. 13.2) coincide quite accurately with the energies of neon levels. Therefore, the conditions for resonant excitation transfer according to the scheme are well realized
At correctly selected pressures of neon and helium, satisfying condition (13.7), it is possible to achieve a population of one or both levels of neon that is significantly higher than that in the case of pure neon, and to obtain an inversion of the populations of these levels with respect to the level.
Depletion of the lower laser levels occurs in collisional processes, including collisions with the walls of the gas-discharge tube.
We emphasize that the method of transferring energy from a gas that does not directly work, but is easily excited, to a gas that does not accumulate excitation energy, but easily emits, which has found wide application in the quantum electronics of gas lasers, was first implemented in a helium-neon laser.
Let us now consider in more detail the level diagram of neutral helium and neon atoms (Fig. 13.3).
The lowest excited states of helium correspond to energies of 19.82 and 20.61 eV. Optical transitions from them to the ground state are prohibited in the -bond approximation valid for helium. States and are metastable states with a lifetime of approximately . Therefore, they accumulate energy well when excited by electron impact.
For neon, a pro-interval -connection is valid. In Fig. In Figure 13.3, states related to one configuration are shown with a thick line highlighting the operating sublevel. To identify the levels, Paschen notations, the most widely used in the existing literature, are used. The levels are close to the metastable levels of helium 250 and 2%, the energy deficit is approximately equal (Note that at 300 K
.) The state has a long lifetime due to the resonant trapping of radiation due to radiative coupling with the ground state.
In neon, s-states have longer lifetimes than p-states. This, generally speaking, makes it possible to obtain an inversion at transitions. It should, however, be borne in mind that the neon state is well populated in the discharge and, if the discharge currents are not too high, stepwise (cascade) population of the lower laser levels is possible during transitions from the state
Rice. 13.3. Diagram of the lower excited energy levels of helium and peon: straight upward arrows - excitation of helium, wavy arrows - transfer of excitation energy from helium to neon, slanted straight arrows - radiation from neon atoms. The relaxation channels of the lower laser levels of neon are not shown.
The introduction of a relatively large amount of helium into the discharge, which provides an intense channel for the population of states external to neon, removes restrictions on the possibility of obtaining inversion in a continuous mode. Historically, generation at the transition was the first to be obtained. The main power corresponds to the transition. Then the inversion of transitions and was implemented.
All three types of generation occur under approximately the same discharge conditions and have the same dependences of the generation power on the discharge parameters. In this case, the competition of generations at waves of 3.39 and 0.63 μm, which correspond to transitions with a common upper level, is especially important. Therefore, generation on one of these waves weakens generation on the other of them. The matter is complicated by the sharp difference in gain factors. The transition corresponds to a gain in and therefore lasing is easily achieved at it in simple, for example metal, mirrors. Transition much
more capricious. It corresponds to a small gain in , which, other things being equal, cannot compete with the gigantic gain in . Therefore, to obtain lasing in the visible region, a helium-neon laser is equipped with multilayer dielectric interference mirrors that have a high reflectivity only at the required wavelength. The transition corresponds to the generation gain achieved. using dielectric mirrors.
The helium-neon laser is a gas-discharge laser. Excitation of helium (and neon) atoms occurs in a low-current glow discharge. In general, in continuous-wave lasers on neutral atoms or molecules, weakly ionized plasma of the positive column of a glow discharge is most often used to create the active medium. The current density of the glow discharge is . The strength of the longitudinal electric field is such that the number of electrons and ions appearing in a single segment of the discharge gap compensates for the loss of charged particles during diffusion to the walls of the gas-discharge tube. Then the positive column of the discharge is stationary and homogeneous. The electron temperature is determined by the product of the gas pressure p and the internal diameter of the tube D. At low temperatures the electron temperature is high, at high temperatures it is low. The constancy of the value determines the conditions for the similarity of the discharges. At a constant density of the number of electrons, the conditions and parameters of the discharges will remain unchanged if the product is constant. The density of the number of electrons in the weakly ionized plasma of the positive column is proportional to the current density. meaning .
For the region of 3.39 µm (series, the strongest line), the upper laser level, as already mentioned, coincides with the upper level of the red lasing line of 0.63 µm. Therefore, the optimal discharge conditions turn out to be the same.
In very common cases, when the same sealed gas discharge tube is used in a helium-neon laser with interchangeable mirrors for operation in different wavelength ranges, some compromise values are usually selected in a fairly wide range of parameters: gas discharge tube diameter 5-10 mm, ratio partial pressures 5-15, total pressure 1 - 2 Torr, current 25-50 mA.
The presence of an optimum diameter is due to the competition of two factors. Firstly, with an increase in the cross-section of the active medium of the laser, all other things being equal, the probability of decay on the capillary wall of the metastar of the gas-discharge tube capillary increases, and the gain increases proportionally. The latter occurs both due to an increase in the probability of decay of the metastable state of neon on the capillary wall and due to an increase in the amount of excited helium (and thereby neon), and therefore the gain while maintaining a constant product, i.e., when performing conditions for the similarity of glow discharges when the diameter of the gas-discharge tube changes.
The presence of an optimal discharge current density is due to the occurrence of cascade processes such as
leading to a decrease in inversion (see Fig. 13.2 and 13.3). Processes of this kind can also become significant with increasing neon pressure, which, in turn, determines the presence of an optimum pressure.
Typical values of the radiation power of helium-neon lasers should be considered tens of milliwatts in the regions of 0.63 and 1.15 microns and hundreds of milliwatts in the region of 3.39 microns. The service life of lasers, in the absence of manufacturing errors, is limited by discharge processes and is calculated in years. Over time, the gas composition changes in the discharge. Due to the sorption of atoms in the walls and electrodes, a “hardening” process occurs, the pressure drops, and the ratio of the partial pressures of helium and neon changes.
Let us now dwell on the issue of designing the resonators of a helium-neon laser. Greater short-term stability, simplicity and reliability of the design are achieved by installing resonator mirrors inside the discharge tube. However, with this arrangement, the mirrors deteriorate relatively quickly in the discharge. Therefore, the most widely used design is one in which a gas-discharge tube, equipped with windows located at a Brewster angle to the optical axis, is placed inside the resonator. This arrangement has a number of advantages - the adjustment of the resonator mirrors is simplified, the service life of the gas discharge tube and mirrors is increased and their replacement is made easier,
it becomes possible to control the resonator and use a dispersive resonator, mode selection, etc.
In quantum electronics, an important question is the width of the working transition line (see lecture two). Natural, collisional and Doppler broadenings are significant for gas lasers. In the case of a helium-neon laser, formula (2.8) (where by means the natural lifetime of the p-state of neon, and by the time t, related to the s-state) gives the value of the natural linewidth MHz. Collisional broadening (formula (2.31) is determined by gas pressure. For neon atoms, under the assumption that the cross section of the corresponding collision process is equal to the gas-kinetic one, at a pressure of the order of MHz. The Doppler linewidth (formula (2.28) is determined, in particular, by the radiation wavelength. For line 0.63 μm at 400 K, these formulas give which is in good agreement with experimental data. From the above it is clear that in the case of a helium-neon laser, the main mechanism causing broadening of the emission line is the Doppler effect. This broadening is relatively small and with such a line it is possible obtain generation on one longitudinal mode, i.e., single-frequency generation with a resonator length of 15 cm, although small but physically feasible (formula (10.21)).
The helium-neon laser is the most representative example of gas lasers. Its radiation clearly reveals all the characteristic properties of these lasers, in particular the Lamb dip, discussed in lecture eleven. The width of this dip is close to the width of one of those uniformly broadened lines, the combination of which forms a nonuniformly broadened Doppler line. In the case of a HeNe laser, this uniform width is the natural width. Since , the position of the Lamb dip (see Fig. 11.6) very accurately shows the position of the center of the working transition line. The curve shown in Fig. 11.6, for the Lamb dip is experimentally obtained by smoothly changing the length of the cavity of a single-mode laser. Consequently, the position of the dip minimum can be used with appropriate feedback controlling the length of the resonator to stabilize the laser generation frequency. This resulted in relative stability and frequency reproducibility equal to . Note, however, that higher stability is achieved when the dip is burned not in the gain line of the active medium, but in the absorption line of the resonant gas. For the generation line, this gas is methane.
Having emphasized in conclusion that there is a whole range of gas lasers based on neutral atoms, including noble gas atoms, we note that the industry produces helium-neon lasers in a wide range.
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.
Gas helium-neon lasers (He-Ne lasers) produced by the German company LSS have a reliable design, good beam quality and a long service life - up to 20,000 hours. The He-Ne laser series is represented by a wide variety of laser models, single-mode and multi-mode, with output powers from 0.5 to 35 mW, emitting in the spectral range of red, green and yellow. There are also Brewster window laser tubes for educational and scientific purposes.
All models are equipped with a power supply. LGK series gas argon ion lasers meet an impressive list of world standards and are certified by CDRH, IEC, CSA, CE, TUV, UL. LSS provides efficient support for its own lasers operating worldwide, providing its customers with a convenient and fast laser tube replacement service. In addition to serial models, the company produces laser systems for individual orders.
The HeNe laser is designed for a wide range of applications such as scanning microscopy, spectroscopy, metrology, industrial measurement, positioning, alignment, directional, testing, code verification, scientific, basic and medical research, as well as educational purposes.
Technical characteristics of laser modules
The tables below summarize the key characteristics of the lasers. For all items below, the specifications listed represent the overall performance of standard models. Individual characteristics can be optimized for specific applications. Please contact our company consultant if you have special requests.
Technical characteristics of laser tubes
Power supply specifications
All models of gas argon ion lasers of the LGK series are equipped with a power supply manufactured by LSS.
- In contact with 0
- Google+ 0
- OK 0
- Facebook 0
