阅读说明：本技术 光抽运的无粒子数反转吸收各向异性二能级增益介质激光器 (Optically pumped two-level gain medium laser without population inversion absorption anisotropy ) 是由 赖天树 于 2020-04-29 设计创作，主要内容包括：发明了一种光抽运的无粒子数反转吸收各向异性二能级增益介质激光器,其结构原理如摘要附图所示。由谐振腔(1,2),光抽运系统(3,5)和增益介质(4)三部分组成；此发明的关键在于二能级增益介质具有强的吸收空间各向异性和发光效率,谐振腔的光轴与增益介质的最弱吸收光的传播方向一致,因而振荡激光具有弱的吸收损耗,而抽运光沿增益介质的最强吸收光的传播方向照射,激发足够的、但非粒子数反转的二能级激发态布居,克服弱的吸收损耗,实现激光辐射。当二能级介质具有强的线二向色性吸收各向异性时,输出激光辐射是线偏振的；当二能级介质具有强的圆二向色性吸收各向异性时,抽运光应该使用使吸收最强的圆偏振态,输出激光辐射也是圆偏振的。(The invention discloses an optically pumped non-population inversion absorption anisotropy two-level gain medium laser, and the structural principle of the optically pumped non-population inversion absorption anisotropy two-level gain medium laser is shown in an abstract figure. The device consists of three parts, namely a resonant cavity (1, 2), an optical pumping system (3, 5) and a gain medium (4); the key point of the invention is that the two-level gain medium has strong absorption space anisotropy and luminous efficiency, the optical axis of the resonant cavity is consistent with the propagation direction of the weakest absorbed light of the gain medium, so that the oscillation laser has weak absorption loss, and the pumping light irradiates along the propagation direction of the strongest absorbed light of the gain medium to excite enough two-level excitation state population which is not particle number inversion, thereby overcoming weak absorption loss and realizing laser radiation. When the two-level medium has strong linear dichroic absorption anisotropy, the output laser radiation is linearly polarized; when the two-level medium has strong circular dichroic absorption anisotropy, the pump light should use the circular polarization state that gives the strongest absorption, and the output laser radiation is also circular polarized.)

1. An optically pumped non-population inversion absorption anisotropy two-level gain medium laser comprises a gain medium, a resonant cavity and an optical pumping system; the gain medium is characterized by being of a two-level structure, having strong absorption space anisotropy and meeting the following conditions:formula (III) α_minAnd α_maxRespectively the minimum and maximum of the anisotropic absorption,andrepresentation of the spherical coordinates, v, for the directions of minimum and maximum absorption, respectively₀Is the resonant absorption frequency of the two-level gain medium; laser oscillation direction and absorption spaceWeakest absorption direction of inter-anisotropic two-level mediumThe irradiation direction of optical pumping is consistent with the strongest absorption direction of the two-level medium with anisotropic absorptionThe consistency is achieved; the non-population inversion laser radiation can be realized under the optical pumping with proper intensity.

2. An optically pumped particle-count-free inversion-line dichroic absorption anisotropic two-level gain medium laser is composed of a gain medium, a resonant cavity and an optical pumping system; the gain medium is characterized by being of a two-level structure, has strong line dichroism and absorbs spatial anisotropy, and meets the following conditions:in the formula P₀And P₁The linear polarization states of light at the weakest and strongest absorption of the linear dichroic absorbing anisotropic medium, respectively, the remaining symbols being as defined in claim 1; laser oscillation direction and the weakest absorption direction of the line dichroism absorption anisotropy two-level mediumThe irradiation direction of optical pumping is consistent with the strongest absorption direction of the line dichroic absorption anisotropic two-level mediumThe consistency is achieved; can realize non-population inversion laser radiation under the pumping of light with proper intensity, and the output laser is P₀The state of linear polarization.

3. An optically pumped particle-number-inversion-free circular dichroism absorption anisotropy two-level gain medium laser is composed of a gain medium, a resonant cavity and an optical pumping system; it is characterized by a gain mediumThe material is a two-level structure, has strong circular dichroism absorption anisotropy, and meets the following conditions:in the formula P_0σAnd P_1σCircular polarization states of light at the weakest and strongest absorption of the circular dichroic absorbing anisotropic medium, respectively, the remaining symbols being as defined in claim 1; laser oscillation direction and weakest absorption direction of circular dichroism absorption anisotropy two-level mediumThe irradiation direction of optical pumping is consistent with the strongest absorption direction of circular dichroism absorption anisotropy two-level mediumThe consistency is achieved; p at appropriate strength_1σRealizing non-particle number reversal laser radiation under circular polarization state optical pumping and outputting P_0σLaser light in a circularly polarized state.

Technical Field

The invention relates to a novel two-energy-level non-particle-number reversal laser transmitter, and belongs to the technical field of laser. The laser emitting method is characterized in that laser emission is realized under the condition of no population inversion by using a two-level structure crystal with absorption space anisotropy and dichroism absorption space anisotropy. The invention opens up a new field of the two-level laser, greatly promotes the development of the laser technology, enriches the types of the laser and the wavelength of the directly generated laser, and the two-level medium is far richer than the three-level or four-level medium.

Background

Laser technology is now developed based on the concept of stimulated emission proposed by einstein, and laser light is obtained by realizing coherent amplification of stimulated emission. Various types of gas, liquid and solid state lasers have been implemented to date. Semiconductor lasers and fiber lasers are derived from solid state lasers. The laser application not only goes deep into our daily life, but also is applied to the great projects of the world and the sea, even the laser intense field can control the atomic nuclear fusion, and the laser controlled nuclear fusion is being developed. However, all lasers developed so far, regardless of their form, have gain media with at least three-level structures and mostly four-level structures, because current laser physics theories suggest that to achieve stimulated radiation amplification, population inversion must be achieved, in other words, population inversion is a necessary condition for achieving laser radiation. Since the two-level structured medium is hard to cause population inversion, the two-level structured medium cannot generate laser light. Therefore, laser development has been in recent 60 years, and a two-level structure laser that stably outputs laser light has not been reported so far. Although a few documents report two-level lasing research, the research is focused on how to realize population inversion in two levels by using a special pumping means, or the observation of two-level resonance lasing spectrum components is reported, and stable laser emission of a two-level system under incoherent white light continuous pumping is never reported. The invention is to break through the limit of the existing laser principle theory cognition: the invention discloses a non-population inversion two-level laser, which is a necessary condition for population inversion.

Disclosure of Invention

Turning to the textbook of laser principles, it can be found that the change of the photon flux density F of a light beam describing the propagation distance dz of the light beam in a two-level medium satisfies the following equation,

dF＝(W₂₁N₂-W₁₂N₁)dz＝F(B₂₁N₂-B₁₂N₁)dz (1)

in the formula W₂₁，B₂₁And N₂Respectively the stimulated emission rate, einstein stimulated emission coefficient and population of the high level 2 in the two-level structure, and W₁₂，B₁₂And N₁Respectively the stimulated absorption rate of the low energy level 1 in the two-energy-level structure, the Einstein stimulated absorption coefficient and the population number.

Einstein recognized as g₂B₂₁＝g₁B₁₂(ii) a The formula is as follows:

dF＝Fg₂B₂₁(N₂/g₂-N₁/g₁)dz (2)

in the formula g₁And g₂The degeneracy of the low energy level 1 and the high energy level 2 in the two-energy-level structure respectively.

Equation (2) shows that to achieve amplification during light propagation, i.e., dF/dz > 0, N must be satisfied₂/g₂＞N₁/g₁Or N is₂＞N₁(when g is₂＝g₁1), this is the population inversion, since under thermal equilibrium conditions, N is always present₂/g₂＜N₁/g₁Or N₂＜N₁(when g is₂＝g₁When 1). Even if the pump is strong, the transparent state can be achieved, and the inversion cannot be achieved. Therefore, the two-level system cannot produce stable laser output. The theoretical theory is concluded, so that experimental researchers do not research the two-level laser; therefore, no realization of a two-level laser has been reported so far.

However, reviewing the source of the above conclusion, it comes from equation (2), and equation (2) is in Einstein relationship g₂B₂₁＝g₁B₁₂Derived from equation (1) under the established conditions. Einstein relationship g₂B₂₁＝g₁B₁₂Is obtained based on the direct interaction of electromagnetic radiation with atoms or molecules of two energy levels under thermal equilibrium conditions. Thermally balanced electromagnetic radiation propagates isotropically, with uniform energy density, polarizedly isotropically, over a wide frequency spectrum. Whereas in practical lasers the optical radiation is propagating unidirectionally, the energy density varies with propagation distance, the spectrum is extremely narrow, and even the polarization can be oriented (typically using brewster windows or cutting the end face of a solid gain medium to the brewster angle). Therefore, the characteristics of optical radiation and thermally balanced electromagnetic radiation in lasers differ greatly. On the other hand, the two-level atoms or molecules that interact with the thermal equilibrium radiation are isotropically randomly oriented; while the orientation of the two-level atoms or molecules in a solid or some liquid medium is locked in a particular direction, which also deviates from the orientation of the two-level atoms or molecules individuallyOriented towards the same polarity randomly. Therefore, in the actual solid laser, the characteristics of both the optical radiation field and the two-level system are greatly different from those of the thermal equilibrium radiation field and the two-level atoms and molecules. Therefore, we have reason to suspect that in practical solid two-level laser cavities the einstein relationship does not hold, i.e. g₂B₂₁≠g₁B₁₂(ii) a Therefore, equation (2) does not hold. However, equation (1) is still true. For a two-level atomic or molecular system in a solid, we believe that it is entirely possible for the excited absorption coefficient B to exist due to the fixed orientation of the atoms and molecules and the lattice field₁₂The complete symmetry in the heat-balanced radiation field, i.e. B₁₂Is the spatial propagation direction of the radiation fieldAs a function of the polarization P and the frequency v, i.e.If it is directed to a particular directionPropagated quasi-monochromatic (v)₀) Specific polarization (P)₀) Is radiated by the radiation source ofAt this time, equation (1) is approximated as:

equation (3) shows that as long as N is present₂If the excited state is more than 0, namely the excited state is populated, the unilaterally transmitted radiation photon flow F can be amplified, and the condition of population inversion does not need to be met: n is a radical of₂＞N₁。

ConditionWhich may be too harsh or severe, the present invention shows that under a more relaxed condition:next, it is possible to realize light amplification. At this time, equation (1) is approximated as:

dF/dz＝F(B₂₁N₂-B₁₂N₁)≥FB₂₁(N₂-N₁/2) (4)

obviously, a quasi-monochromatic light flux density F with a specific polarization, transmitted in a specific direction, is to obtain a gain, as long as N is₂＞N₁2 instead of N₂＞N₁The population inversion condition of (1). In other words, as long as N is present₂＞N/3(N＝N₁+N₂Total population of two-level atoms), or more than one third of the atoms are in excited state 2, a gain can be obtained by a photon stream propagating unidirectionally through the two-level system. And the population-separation saturation transparent condition N/2 of N/3 has a quite wide adjustable range, so that the gain adjustable range in the two-level system is quite large, completely practical and has practical value. Therefore, the invention has great application value.

In fact, B₁₂The anisotropy of (2) has been actually reported in reality. In some stressful or nanostructured crystals, the phenomenon of absorption anisotropy is reported, i.e. the absorption coefficients are different for different directions of light transmission. While absorption-dependent polarization has been reported earlier, the so-called linear dichroic effect and circular dichroic effect are just polarization-dependent effects of absorption. Therefore, only the stimulated absorption coefficient B of the two-level solid medium₁₂Is so strong as to satisfyUnder the conditions, the two-level luminous medium is suitable for manufacturing a two-level laser.

However, one practical problem is thatAnd B₂₁Are all quantities that are difficult to measure experimentally directly, so that the conditions cannot be judged practically:to this end, the invention proposes an alternative condition:

in the formula, alpha is the absorption coefficient of the two-level medium and can be measured through experiments. Equation (5) requires that the maximum value of the anisotropic absorption coefficient of the two-level absorption anisotropic laser medium be at least 100 times greater than the minimum value. It is similar to the FOM parameter of a three-level or four-level laser crystal. The FOM of a three or four level laser crystal is generally required to be more than or equal to 100.

The anisotropic absorption coefficient α is experimentally measurable, narrow-band quasi-monochromatic and linearly polarized light absorbed by two-level resonance can be used for continuously rotating the polarization direction of a linear polarization surface through a 1/2 wave plate, the linearly polarized light can be converted into left-handed or right-handed circularly polarized light through a 1/4 wave plate, the two-level crystal to be measured is cut into polyhedrons and installed on a three-dimensional rotating platform, and the measuring crystal is used for measuring different light transmission directionsAbsorption coefficient of narrow-band quasi-monochromatic light with different polarization states P (linear polarization, left-handed and right-handed circular polarization)The maximum absorption coefficient obtained by the measurement isMinimum absorption coefficient ofIf they satisfy equation (5), the two-level anisotropic crystal is suitable for manufacturing a laser, and two-level non-population inversion laser emission can be realized.

Drawings

FIG. 1 schematic diagram of an absorption space anisotropy two-level laser

FIG. 2 schematic diagram of a two-level laser with dichroic absorption space anisotropy

FIG. 3 schematic diagram of a circular dichroic absorption space anisotropic two-level laser

In fig. 1, 1 is a high reflectivity mirror as a total reflection mirror of a laser cavity; 2, a partially-transmitting reflector used as a coupling output mirror of the laser; 3 is a pumping light source; 4 is a two-level laser crystal having an absorption spatial anisotropy; the optical axis direction of the laser is the weakest absorption direction, namely the laser oscillation is along the weakest absorption direction, so that the absorption loss is minimum; 5 is a light energy collecting and directional transmitting system, which directionally transmits the energy emitted by the 3 to the crystal 4 to excite enough excited state particles N₂Realizing the amplification of the stimulated radiation; the direction of the illuminating light is along the direction of strongest absorption of the crystal 4. 6 is the output laser beam; Φ is the angle between the direction of the weakest absorption and the direction of the strongest absorption of the crystal 4.

In fig. 2, 1 is a high reflectivity mirror as a total reflection mirror of the laser cavity; 2, a partially-transmitting reflector used as a coupling output mirror of the laser; 3 is a pumping light source; 4 is a two-level laser crystal having linear dichroic absorption spatial anisotropy, i.e. absorption depends not only on the direction of propagation of the light, but also on the direction of linear polarization of the light. The optical axis direction of the laser is the direction with the weakest line dichroism absorption, namely the laser oscillation is along the weakest absorption direction, so that the absorption loss is minimum; 5 is a light energy collecting and directional transmitting system, which directionally transmits the energy emitted by the 3 to the crystal 4 to excite enough excited state particles N₂Realizing the amplification of the stimulated radiation; the direction of the illuminating light is along the direction of the strongest line dichroic absorption of the crystal 4. 6 is the output laser beam; due to the strong line dichroic absorption of the crystal, the output laser beam should be linearly polarized; a linear polarizer 7 for converting the light emitted from the light source 3 into linearly polarized light; the light passing direction of the linear dichroic absorber can be rotated until the polarization direction is consistent with the strongest linear dichroic absorption direction. Φ is the angle between the direction of the weakest absorption and the direction of the strongest absorption of the crystal 4.

In fig. 3, 1 is a high reflectivity mirror as the total reflection mirror of the laser cavity; 2, a partially-transmitting reflector used as a coupling output mirror of the laser; 3 is pumping lightA source; 4 is a two-level laser crystal having circular dichroic absorption spatial anisotropy, i.e. absorption depends not only on the direction of propagation of the light, but also on the handedness (left or right) of the circular polarization of the light. The optical axis direction of the laser is the direction with the weakest circular dichroism absorption, namely the laser oscillation is along the weakest absorption direction, so that the absorption loss is minimum; 5 is a light energy collecting and directional transmitting system, which directionally transmits the energy emitted by the 3 to the crystal 4 to excite enough excited state particles N₂Realizing the amplification of the stimulated radiation; the direction of the irradiated light is along the strongest direction of circular dichroism absorption of the crystal 4; 6 is the output laser beam; the output laser beam is circularly polarized due to the strong circular dichroic absorption anisotropy. A linear polarizer 7 for converting the light emitted from the light source 3 into linearly polarized light; 1/4 wave plate for converting incident linearly polarized light into circularly polarized light; the fast and slow axes of the pump can rotate freely to obtain left or right circularly polarized light, so that the strongest absorption of the pump light is realized. Φ is the angle between the direction of the weakest absorption and the direction of the strongest absorption of the crystal 4.

Detailed Description

Based on the above summary, the present invention has been embodied in three examples that produce unpolarized, linearly polarized, and circularly polarized laser outputs, respectively.

Example one, absorption spatial anisotropy two-level laser

If the absorption anisotropy of a two-level crystal is so strong as to satisfy equation (5), and has a strong luminous efficiency. The two-level laser crystal is suitable for being used as a laser. The crystal is cut into parallel tetrahedrons or cylinders, so that the direction of the weakest absorption is consistent with the direction of the optical axis. The laser shown in the schematic diagram of fig. 1 is formed by the resonant cavity, the pumping light source and the light collection system. The mirrors 1 and 2 form a resonant cavity, and the laser light is coupled out from the mirror 2. 4 is the cut absorption anisotropy two-level laser crystal, namely the gain medium. And 3 is a pumping light source, and 5 is a light collecting and directional transmission system, so that pumping light irradiates the laser crystal along the strongest absorption direction of the crystal 4. The laser is pumped in the direction of strongest absorption and oscillates in the direction of weakest absorption, thus enabling a net energy accumulation that translates into optical gain. When the pumping power of 3 exceeds the laser oscillation threshold, stable laser light can be output.

Example two, linear dichroic absorbing spatially anisotropic two-level laser

If a two-level crystal has a linear dichroic absorption anisotropy, and this absorption anisotropy is so strong as to satisfy equation (5), and has a strong luminous efficiency. The two-level laser crystal is suitable for being used as a linear polarization laser. The crystal is cut into parallel tetrahedrons or cylinders, so that the direction of the weakest absorption is consistent with the direction of the optical axis. The laser shown in the schematic diagram of fig. 2 is formed by the resonant cavity, the pumping light source and the light collection system. The mirrors 1 and 2 form a resonant cavity, and the laser light is coupled out from the mirror 2. And 4, the cut line dichroic absorption anisotropic two-level laser crystal, namely the gain medium. And 3 is a pumping light source, and 5 is a light collecting and directional transmission system, so that pumping light irradiates the laser crystal along the strongest absorption direction of the crystal 4. The linear polarizer 7 is rotated in orientation to maximize the absorption of the pump light. The laser is pumped in the direction of strongest absorption and oscillates in the direction of weakest absorption, thus enabling a net energy accumulation that translates into optical gain. When the pumping power of 3 exceeds the laser oscillation threshold, stable laser light can be output. Due to linear dichroic absorption, the output laser light should be linearly polarized. The polarization direction is along the polarization direction where dichroic absorption is the smallest.

It is noted that the linear polarizer 7 in this example may be eliminated, and instead the pumping efficiency of the pumping light may be increased. Since natural light contains two orthogonally polarized, equal-intensity components. The polarizer 7 removes a component orthogonal to its polarization pass direction. However, the removed polarization component, although weakly absorbed, is also partially absorbed, thereby enhancing the pumping efficiency.

EXAMPLE III, circular dichroism absorption space anisotropy two-level laser

If a two-level crystal has circular dichroic absorption anisotropy, and this absorption anisotropy is so strong as to satisfy equation (5), and has strong luminous efficiency. Such a two-level laser crystal is suitable as a laser for generating circularly polarized laser light. The crystal is cut into parallel tetrahedrons or cylinders, so that the direction of the weakest absorption is consistent with the direction of the optical axis. The laser shown in the schematic diagram of fig. 3 is formed by the resonant cavity, the pumping light source and the light collection system. The mirrors 1 and 2 form a resonant cavity, and the laser light is coupled out from the mirror 2. And 4, a cut circular dichroism absorption anisotropy two-level laser crystal, namely a gain medium. And 3 is a pumping light source, and 5 is a light collecting and directional transmission system, so that pumping light irradiates the laser crystal along the strongest absorption direction of the crystal 4. The orientation of the linear polarizer 7 is fixed, and the fast and slow axes of the 1/4 wave plate 8 are rotated to generate left-handed or right-handed circularly polarized pump light, so that the absorption of the pump light reaches the maximum value. The fast and slow axes of 1/4 wave plate 8 can be fixed, and the left or right circularly polarized light can be obtained by rotating the orientation of linear polarizer 7. The laser is pumped in the direction of strongest circular dichroism absorption and oscillates in the direction of weakest absorption, thus enabling a net energy accumulation that translates into optical gain. When the pumping power of 3 exceeds the laser oscillation threshold, stable laser light can be output. The output laser light should be circularly polarized due to circular dichroic absorption. The handedness coincides with the handedness at which circular dichroism absorption is minimum.