RU2239920C1 - Method for organizing inversion distribution in laser active element - Google Patents

Abstract

FIELD: quantum electronics for laser technology, data transfer systems, medicine, and research work.

SUBSTANCE: proposed method used for solid state , liquid, and gas lasers includes organization of inversion distribution in laser active element of length 1 in direction of z, 0 < 1 < z, with characteristic size of input aperture a ₀ along transverse coordinate r, while selectively pumping active element by beams in narrow spectral domain Δλ _p near wavelength λ _p , Δλ _p << λ, into medium absorption band in infrared, visible, or ultraviolet spectrum range. Pumping beam having characteristic transverse size r ₀ and wavefront curvature radius ρ passes through aperture of pumping energy concentration system, then advances in active element in direction z, and is absorbed in active medium thereby organizing inversion of n(r, z). Diaphragm having transverse size a, a < r ₀ < a ₀ is proposed for use as energy concentration system; active element is placed in Fresnel beam diffraction region on diaphragm so that integer Fresnel number N = a ² /λZ ≥ 1, where Z = zρ((z + ρ), corresponds to distance z between diaphragm and input aperture of active element; active element length is chosen from expression l ≤ a ² /λN(N + 1).

EFFECT: enhancing pumping power density in active medium by many times.

1 cl, 5 dwg

Description

The invention relates to quantum electronics and can be used when working with solid-state, liquid and gas lasers used in laser technology, information transfer systems, medicine, in scientific research.

Known are traditional methods of forming the inversion distribution in the active elements of solid-state, liquid, and gas lasers when they are selectively pumped by radiation beams in the IR, visible, or UV spectral ranges / 1-12 /. Sources of selective pumping can be semiconductor laser diodes, neodymium glass lasers and neodymium garnet crystals, and other types of lasers / 1-12 /. The pump radiation in a narrow spectral region Δ λ _p near the wavelength λ, Δ λ _p << λ, is directed into the active medium with an absorption bandwidth Δ λ _a and with an absorption coefficient k (λ). Usually, Δ λ _p <Δ λ _a , and the length of the medium 1 along the pump direction z, 0 <l <z, is chosen such that its optical thickness, k (λ) 1≥ 3. At a pump power density I (r) less the saturation power density of the absorption band, I _us , this condition provides almost complete absorption of the pump in the medium / 3 /. For I (r)> I _us , the pump radiation enlightens the active medium and partially leaves it without being absorbed / 3 /. A variety of selective pumping schemes are used, which differ in the orientation of the pump propagation directions in the medium with respect to the direction of the working laser beam. Known schemes are “transverse” - the direction of pumping perpendicular to the working laser beam and “longitudinal” - pumping along the laser beam / 1-12 /. Practically in each of the traditional schemes, one or another system of concentration of the pump radiation on the active element is used, which may include reflectors, optical fibers, optical focusing radiation / 1-12 /. On figa presents a diagram of the "longitudinal" pump with focusing radiation in the active element / 7-12 /. The numbers in FIG. 1a indicate: 1 — pump beam; 3 - lens focusing the pump radiation; 4 - active element. A pump beam with a characteristic size r ₀ along the transverse coordinate r, passing through the aperture of the concentration system (objective), enters the input aperture of the active element with a size a ₀ , r ₀ ≤ a ₀ , and then propagates in the active medium along the z direction, FIG. 1a. The inversions occurring during pumping in the active medium, n (r, z), depend on the distribution of the pump intensity in the beam cross section, on the concentration system used, and on the absorption coefficient k (λ). In Fig. 1a, the initial pump beam is close to parallel, i.e. the radius of curvature of its wavefront is ρ ≈ ∞. For pumping, of course, beams with a certain angular divergence can also be used, for which ρ takes on specific (positive) values. For convergent beams, ρ are negative.

Many pump schemes are oriented toward obtaining a nearly uniform distribution of inversion over the volume of the active medium. Such distributions are often used in both generators and laser amplifiers. The transverse pumping of a LiF: F crystal was applied in / 4 / $\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$ with an aperture of 80 cm ² and a thickness of 1 = 4 cm by radiation from a neodymium glass laser with an energy of 700 J. In / 5 /, the amplifier was longitudinally pumped on a plate made of a Yb: S-FAP crystal with an aperture of 6 × 6 mm and l = 44 mm radiation from a Cr: LiSAF crystal laser, and / 6 / used several batteries of semiconductor diodes with a total power of 80 kW to pump 8 plates of this crystal with dimensions of 4 × 6 × 0.75 cm. To concentrate the pump radiation on the active elements in / 4-6 /, mirrors, lenses, and optical fibers were used. In works / 7, 8 /, longitudinal pumping of active elements (LiF: F crystal was used $\genfrac{}{}{0ex}{}{\text{+}}{\text{2}}$ / 7 / and ytterbium-erbium glass / 8 /) by the radiation of a neodymium laser. To concentrate the pump radiation on the active elements, the pump beam was compressed using the telescopic system / 8 / or the location of the active medium in the caustic of a telephoto lens / 7 / was used. In [4–8 /, in the active elements, a nearly uniform distribution of pumping and inversion was formed.

One of the widely used selective pumping schemes is also the focusing of the radiation beam with the lens into the volume of the laser medium in the cavity, and in the focal region of the lens they try to form an inversion distribution corresponding to the main type of resonator vibrations. In this case, favorable conditions are created for obtaining single-mode oscillations in this resonator in this type of oscillations. Such pumping circuits are described in / 9-12 /.

In the known selective pumping schemes, it is quite difficult to form inverse cross sections of the active medium with several maxima and minima that would correspond to the conditions of selective excitation in the generator, not of the main one, but of one of the highest modes of amplification or amplification conditions in the active medium of laser beams with a profiled intensity distribution . At the same time, the formation of such inversion distributions in the active medium for obtaining single-mode lasing at higher types of oscillations and for amplifying profiled laser beams is of considerable interest for many applications of lasers. Of interest is also the development of new methods for concentrating pump radiation on active laser elements.

Closest to the invention is a method of forming the distribution of inversion in the active element on a Yb: YAG crystal, described in / 10 /. A collimated pump beam with a diameter of about 10 mm from a laser on a sapphire crystal with titanium operating in a narrow spectral range, Δ λ _p ≈ 1 nm, near a wavelength of λ ≈ 0.94 μm, passed through an aperture of a lens with a focal length of 150 mm and focused on a plate of Yb: YAG in the cavity. The absorption bandwidth of a Yb: YAG crystal with a center near a wavelength of 0.94 μm, Δ λ _a > 10 nm. An almost complete absorption of the pump was ensured in a plate with a thickness of 1 = 2 mm. The focus area of the pump radiation in the plate well overlapped with the volume of the active medium occupied by the TEM ₀₀ mode, which provided the conditions for single-mode generation of an ytterbium laser at a wavelength of 1.03 μm. When focusing the pump beam in the volume of the active medium, as a rule, a bell-shaped intensity distribution is formed that is characteristic of the far (Fraunhoferophae) zone of propagation of the pump beam. Under these conditions, the main type of oscillations with maximum intensity on the axis receives predominant development in the resonator, which is demonstrated by the results of / 10 / and other publications / 9, 11, 12 /.

The technical task of the invention is to provide a method for generating the inversion distribution in the active element of the laser, which would provide a concentration of pump radiation on the active element and obtain, in the active medium, along with the traditional bell-shaped intensity distribution in the cross section of the pump beam, profiles of a more complex form, including profiles with several maxima and the minima that would correspond to the excitation conditions in the laser cavity of not the main but higher types of vibrations, and for the amplifier, pits for transporting profiled laser beams.

The problem is solved as follows. A method is proposed for forming an inversion in an active element of a laser based on a solid-state, liquid, or gaseous medium with a length of 1 in the z direction, 0 <l <z, with a characteristic input aperture size a ₀ along the transverse coordinate r, including selective pumping of the active element by radiation in a narrow spectral region Δ λ _r approximately λ wavelength, Δ λ _p << λ, in the absorption medium band in the IR, visible and UV spectral ranges, wherein the pump beam to the characteristic transverse dimension r ₀ and a radius of curvature ρ wavefront passes through aperture ICI emy pump energy concentration and then propagates in the active element along the direction z, absorbed in the active medium and forming an inversion distribution n (r, z). A diaphragm with a transverse dimension a, а≤ r ₀ ≤ а ₀ is proposed as a radiation concentration system, the active element is placed in the beam Fresnel diffraction zone on the diaphragm, so that the distance z from the diaphragm to the input aperture of the active element corresponds to an integer Fresnel number N = a ² / λ Z≥ 1, Z = zρ / (z + ρ), and the length of the active element is selected in accordance with the ratio 1≤ a ² / λ N (N + l). A diagram of the formation of the inversion distribution in the active element of the laser using Fresnel diffraction of a pump beam on a circular diaphragm (2) is shown in Fig. 1b.

In the inventive method, it is proposed to use for the formation of inversion profiles in the active medium the distribution of radiation intensities that occur in the near (fresnel) zone of diffraction of the laser beam (pump beam) on the aperture diaphragm. As is known, see, for example, / 13 /, in the case of diffraction of a radiation beam with a wavelength λ on a circular aperture with a radius a <r ₀ , where r ₀ is the beam radius, intensity distributions arise along the beam propagation path along the z axis along maxima alternating along z and lows. The positions of these extreme points on the z axis, as well as the number of maxima (minima) in the beam cross section for a given z coordinate, are determined by integer values of the parameter N = a ² / λ Z (Fresnel numbers). For a collimated pump beam, Z = z. Figure 2-4 presents the calculated curves of the intensity distribution along the coordinates r and z for the collimated pump beam at a wavelength of 0.53 μm, diffracted on a circular aperture ⌀ 1 mm The initial intensity distribution in the beam cross section at the diaphragm was considered uniform with the initial level I ₀ = 1. Figure 2 illustrates the characteristic of Fresnel diffraction on a circular diaphragm, the location of the maxima and minima of intensity along the z axis. The maximum width along z has a maximum corresponding to N = 1. As N grows, the width of the maxima (minima) and the distance between them along the z axis decrease. Choosing the length of the active element 1 in accordance with the change in the Fresnel number, 1≤ a ² / λ N (N + l), you can position the active element within the width of a specific maximum (minimum) of intensity and form within the aperture of the active element with a characteristic diameter a _{0 ≥} r ₀ the distribution of the pump intensity corresponding to a given N. During Fresnel diffraction of converging (or diverging) beams, the diffraction zone shrinks (lengthens), which can be used to solve specific problems of generating inversion distributions in active elements.

An essential circumstance for pumping the active medium is the fact that on the propagation axis of the diffracted beam, and also close to the axis in the Fresnel zone, it is possible to obtain an increase in the initial radiation intensity (power density) I ₀ up to 4 times (for a circular diaphragm, with a uniform intensity distribution I ₀ at its entrance). When N = 1, Fig. 3, the pump distribution has a maximum on the axis, I = 4 I ₀ . At N = 2, Fig. 4a, a distribution is obtained with a minimum on the axis and two maxima with an intensity of ≈ 2 I ₀ . At N = 3, Fig. 4b, the distribution with three maxima: central with an intensity of 4 I ₀ and two lateral ≈ 1.5 I ₀ , etc.

The inversion profiles formed in the active element at different values of N also depend on the pump power density. In order to obtain inversion distributions in the active medium similar to pump intensity distributions in the Fresnel zone of diffraction of the pump beam, the restriction on the power density of the radiation incident on the medium is essential. The density of the pump power I should not exceed the level of the density of the saturation power I _us of the absorption band of the active medium. Under these conditions, as the calculation shows, the inversion profiles practically repeat the distribution of the pump intensity. The distributions shown in Figs. 3 and 4 show (along the ordinates) the values of the inversion density averaged over z, n (r), which can be achieved in a YAG: Nd ³⁺ crystal when it is pumped by radiation at a wavelength of 0.53 μm in the fresnel zone of diffraction of the pump beam at a diaphragm ⌀ 1 mm. For a crystal with an initial activator concentration of ≈ 5 × 10 ¹⁹ cm ^-3 at pump intensities <50 kW / cm ² at N = 1, the inversion density n (0) ≈ 10 ¹⁸ cm ^-3 can be achieved on the beam axis. When I> I inversion distribution _we will deform (flatten) the active medium due to bleaching effect. However, when I> I _have encountered in the inversion distribution medium may be of interest for forming laser beams of profiles in oscillators and amplifiers.

Another essential requirement for the pump parameters is the limitation on the width of the radiation spectrum, Δ λ _p . The pump radiation spectrum should “fit” within the absorption bandwidth of the active medium Δ λ _a , usually Δ λ _p ≤ Δ λ _a . In addition, in order to exclude deformation of the pump distribution profile (and the inversion profile) associated with the superposition of various diffraction intensity distributions resulting from a change in the pump radiation wavelength, it is easy to show that the condition Δ λ _p << λ is fulfilled. For most cases of selective pumping of active media under consideration, this inequality is well satisfied.

In addition to the diffraction intensity distributions with circular symmetry (Fig.2-4) are well known intensity distribution in the Fresnel zone when the diffraction of beams on rectangular, elliptical and a number of other diaphragms / 13 /. These distributions can also be used to form inversion profiles in active media.

The advantage of the proposed method of forming the inversion is its simplicity. Despite some energy loss associated with aperture, no other optics are required to increase the pump energy density except the diaphragm itself. An essential point in the proposed method is also the possibility of its application for the concentration of pump energy in the short wavelength range of wavelengths, where conventional optics cannot be used. It seems attractive to use the proposed method of forming an inversion for microlasers. For small beams, the length of the Fresnel diffraction zone is small (for example, at λ = 1 μm, N = 1, a = 0.1 mm, z = 1 cm) and the pump source together with the active medium can be placed within a small-sized scheme .

The proposed method of forming an inversion in the active element of the laser was confirmed experimentally. The experiment was conducted according to the scheme figb. A beam of radiation at a wavelength of 0.53 μm from a YAG: Nd ³⁺ crystal laser with conversion to the second harmonic was directed to an active element made of YAG ceramic: Nd ³⁺ ⌀ 8 × 40 mm, mounted in a resonator of two mirrors with reflection coefficients ≈ 15% at a wavelength of 1.06 μm. The pump laser operated in the mode of single pulses with a duration of 3.5 ns and an energy of up to 200 mJ. When such a short pulse of radiation is pumped, an inversion in the active medium is also formed in a short time and a “giant” generation pulse develops in the resonator. A pump beam of ⌀ 6 mm could be limited by an iris diaphragm, which was located at a distance of ≈ 50 cm from the active element. It was found that with a decrease in the diameter of the iris diaphragm (i.e., with a decrease in the pump energy) there is an increase in the generation energy of a ceramic laser, a decrease in the duration of the generation pulse and a decrease in the delay time of this pulse relative to the pump pulse. For example, when using a diaphragm ⌀ 2 mm to limit the pump beam, the generation energy did not exceed 3 μJ, the pulse duration was ≈200 ns, and its delay was ≈ 350 ns. At the same time, for a diaphragm ⌀ 1 mm, the generation energy of a ceramic laser reached 130 μJ, the pulse duration and its delay were ≈ 20 ns. Thus, with a decrease in the pump energy, a sharp increase in the energy and lasing power of the ceramic laser was observed. This paradoxical, at first glance, fact is explained by taking into account the features of the intensity distribution in the cross section of the pump beam during its diffraction by the aperture diaphragm. Indeed, for a diaphragm ⌀ 1 mm at λ = 0.53 μm at a distance z = 47 cm from it, there is a maximum of the intensity distribution (N = 1) with a peak value on the z axis ≈ 4 times higher than the intensity of the irradiation pump laser. The active element is in the most favorable region for the formation of a high level of inversion of the Fresnel region of diffraction of the pump beam — in the zone of an extended maximum at N = 1. This leads to an increase in the energy and power of the “giant” pulse emitted by the ceramic laser and a reduction in the delay time of the generation. At the same time, for a diaphragm ⌀ 2 mm, the active element is in the zone of the pump minimum (N≈ 4), the inversion level in the medium is low and the energy and power of the “giant” pulse are small, and the generation delay time is long. The experiment confirms the possibility of concentration of pump radiation in the active element of the laser when it is located in the Fresnel zone of diffraction of the pump beam on the aperture diaphragm bounding the beam. Direct measurements of the distribution of the pump intensity in the Fresnel diffraction zone at N = 1 using an array photodetector showed its correspondence to the calculated values, Fig. 3.

The level of concentration of the pump energy and the nature of the inversion distribution in the medium depend on the selected location of the active element (Fresnel number). A comparison of the inversion distributions shown in Figs. 3 and 4 with the structure of the radiation field of circular types of oscillations of a laser with plane mirrors / 14 / shows that these distributions can be used to select the transverse modes of the laser cavity. The inversion distribution at N = 1 can be used for selection of the main type of TEM ₀₀ oscillations, at N = 2, for the selection of the TEM mode $\genfrac{}{}{0ex}{}{\text{*}}{\text{01}}$ , for N = 3 - TEM ₁₀ modes, etc. When using non-circular apertures (rectangular, elliptical, etc.) to limit the pump beam, it will be possible to form inversion profiles to maintain the predominant development of other types of transverse modes in the laser cavity.

Literature

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Claims (2)

1. The method of forming the inversion distribution in the active element of the laser with a length of 1 in the z direction, 0 <1 <z, with a characteristic input aperture size a ₀ along the transverse coordinate r, including selective pumping of the active element by radiation in a narrow spectral region Δλ _p near the wavelength λ , Δλ _p << λ, into the absorption band of the medium in the IR, visible or UV ranges of the spectrum, and a pump beam with a characteristic transverse size r ₀ and with a radius of curvature of the wavefront ρ passes through the aperture of the pump energy concentration system and propagates then in the active element along the z direction, being absorbed in the active medium and forming the inversion distribution n (r, z), characterized in that the pump beam passes through the diaphragm with a transverse dimension a, and <r ₀ <a ₀ , the active element is placed in the zone Fresnel diffraction of the beam at the diaphragm so that the distance z from the diaphragm to the input aperture of the active element corresponds to a Fresnel integer N = a ² / λZ≥1, where Z = zρ / (z + ρ), and the length of the active element is selected from the relation 1≤ a ² / λN (N + l). 2. The method of forming the inversion distribution in the active element of the laser according to claim 1, characterized in that the power density of the pump beam I <I _us , where I _us is the density of the saturation power of the absorption band of the active medium.

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