Method and device for forming powerful laser pulse

FIELD: laser technology.

SUBSTANCE: method and device can be used at powerful multichannel laser installations with radiation power level higher than 10¹² W. Operation of laser is based upon forming of partially coherent laser pulse. Characteristics of laser pulse are specified by design mutual coherence function. Values of the function are necessary and sufficient for optimal matching of laser-target system. Device for forming powerful laser pulse has master oscillator, system for forming space-time characteristics of laser pulse and amplifiers. Amplifiers are disposed in sequence along rising cross-section of beam to be amplified correspondingly to space-angle distribution of radiation intensity. Distribution is matched with design function of mutual coherence. High uniformity intensity distribution is provided in focus of laser.

EFFECT: simplified design; reduced sizes; improved efficiency of inversion reading; reduced cost of laser energy.

2 cl, 1 dwg

The invention relates to laser technology.

The invention can be used to create powerful multi-channel lasers with power levels of radiation >10¹²W.

There is a method of forming an intense laser pulse on the basis of coherent radiation (Install Dolphin, N.G. Basov and others "Proceedings of the Lebedev physics Institute", 1978, 103, 3-51). In the installation of the system was used in a series-parallel placement amplifier channels with similar active core elements of Nd-glass brand GLS-1 (with a diameter of 45 mm and a length of the pumped part 600 mm). Increase energy and improve spatial and angular characteristics of the laser radiation was carried out by spatially dividing the beam and then hardened. The disadvantage of this method, when used volume of active material is low density, output power ˜1 GW/cm², which was limited by the threshold of the development of self-focusing.

There is also known a method of forming an intense laser pulse on the basis of coherent radiation using a spatial filter beam expanders (The Shiva Laser-Fusion Facility, Speck D.R. et al. "IEEE J. of Quant. Electron.", 1981, QE-17, 9, 1599-1619.) Installation of "Shiva" includes 20 parallel amplifier channels, each of which consists of a rod and disk amplifiers, insulating ele is having spatial filters. The output aperture of 200 mm at the input - ⊘26 mm total power of the focused radiation ˜30 TW. The main disadvantage of this method is practically impossible to achieve the required uniformity of irradiation of spherical targets. This is because in the long amplification system laser pulse passes through dozens of different optical elements, which, in turn, even at high processing quality optical surfaces leads to significant aberrations of the wave front. In addition, the beam quality is influenced by phenomena such as interference and diffraction radiation, small-scale self-focusing beams in optical media, etc. used in the amplifying path special optics allows to improve the structure of the wavefront of the focused radiation and agree on the width of the intensity distribution in the focus of the lens with the size of the target. However, on the surface of the target is formed speckle structure, fixed for the duration of the pulse and fatal methods of linear optics.

Most of the current and newly created powerful lasers based on the use of coherent radiation (Laser fusion device, N.G. Basov and other technology, 1984, vol 25, part 1.).

the main disadvantages of the devices on the basis of coherent light are:

1. The practical impossibility of achieving the desired uniformity of irradiation of the target, as a consequence of the heterogeneity of the distribution of the radiation intensity in the laser focus.

2. The necessity of using complicated devices for suppression of small-scale self-focusing in optical medium of the laser.

3. The impossibility of removing the speckle patterns from the target surface.

4. The design complexity and high cost per unit of laser energy.

The problem solved by the invention is the creation of a laser system capable of forming a highly homogeneous intensity distribution in the laser focus, while simplifying the design, reducing the size, increasing the efficiency of removal of inversion and reducing the unit cost of laser energy.

To solve this problem, a method of forming a powerful laser pulse, based on the use of partially coherent laser radiation, the characteristics of which are specified function of the mutual coherence G(x₁x₂, t), values which are necessary and sufficient to ensure a homogeneous distribution of the output intensity and focus of the laser. When using partially coherent laser pulse in the optical medium of the laser is suppressed interference and diffraction phenomena and, as a consequence, the fine is stubna self-focusing of laser beams, limiting specific power output with the active medium of the laser.

The possibility of creation of such lasers is based on the fact that almost all current and newly created facilities for laser fusion coherence radiation redundant. Indeed, a coherent beam of laser radiation at the focus can be concentrated into a spot whose size is determined by the radius of the circle Erie, ie,

where λ - wavelength radiation. This shows that when the lens aperture is D/F˜1/10 λ˜1 μm, the radius of the spot focus is ˜12 μm, which is much smaller than thermonuclear target (˜1 mm). Therefore, may reduce the degree of spatial coherence of the radiation by a factor of d_m/d_Fthat leads to more effective coordination of the system parameters, the laser target, where d_m/d_F- the ratio of the size of the target and the focal spot.

The expression for the function of mutual coherence G[x₁, x₂, t] laser pulses of duration ˜ 10^-9s, can be represented in the form

where the degree of spatial coherence γ₁₂=λ/(α·D) the size of the coherence, related to the aperture of the beam, and the degree of temporal coherence pulse is Lisa γ _τ=λ²/(δλ··τ) - the coherence time, normalized by the pulse duration τ; α - the divergence of the laser beam, δλ - width line generation, C is the speed of light. In the approximation of linear optics, the degree of spatial coherence of the laser beam γ₁₂determines the characteristic spatial scale of heterogeneity amplitude of the radiation field, and for the case of partially coherent radiation 0<γ₁₂<1. The degree of temporal coherence pulse γ_τis the degree of averaging of the anisotropy field amplitude for a time equal to the pulse duration τ.

Analyze the maximum possible values of the degree of spatial coherence γ₁₂allowed for implementation on the target surface density flux q_m. The expression for the flux density in the focus of the lens with aperture (D/F has the form

where_l- brightness laser beam. After a simple transformation, we get the formula for the allowed values of the degree of coherence in the form

where q_Lthe flux density of the laser radiation output.

Parameter γ₁₂plays a critical role in lasers for LTS. First, it is its value determines the I characteristic spatial scale of the inhomogeneity of the flux density of radiation on the target Δ x˜α·F·γ₁₂and, secondly, its change is achieved the ability to control the intensity distribution in the plane of the target. For example, for q_m=10¹⁴W/cm²when the diameter of the output aperture of the amplifier system 60 mm, the degree of coherence of the beam can be reduced to a value of 2·10^-2. When the pixel array to control the intensity distribution within the spot focus is 2.5·10³. Increasing the aperture formed beams leads to the empowerment of management intensity distribution.

The mechanism for the development of small-scale self-focusing in optical medium is as follows. When distributing the powerful light wave in a heterogeneous environment occurs scattered radiation, the intensity and the angular range which is defined by density inhomogeneities and their characteristic sizes. Field coherently scattered radiation relative to the radiation of the laser pulse, which leads as a result of their interference interaction to spatial modulation of the amplitude of the field strong waves in a wide range of spatial frequencies of the perturbations. During the propagation of a laser pulse in a medium with nonlinear refractive index amplitude perturbations and(x_m) increases differently for different space the single frequency x _m. The most intensely reinforced some selected spatial scale of disturbance and(x_M). The growth of perturbations in this case, when reaching their amplitude certain critical value, leading to the development of small-scale self-focusing of the laser beam, the formation of filamentous destruction of the active medium of the laser and complete degradation of the laser beam.

During amplification of partially coherent radiation pattern of effects differs significantly. First, a strong wave is not δfunction, and some curve I(θ), the width of which is comparable with the spectrum of the scattered radiation. Secondly, the pattern of interaction of two waves fickle neither in time nor in space. And, finally, the interference of the interaction of the scattered radiation is possible only within the size of the coherence strong waves.

Temporal coherence γ_τdoes not exert any substantial influence on the spatial distribution of the field amplitude of the laser radiation. The only thing that may show the limited coherence length L_when=λ²/δλis the interaction of the scattered radiation, propagating at large angles to the beam axis, with a field of strong waves. In this case, if the observation point difference of the rays will exceed the length to which parentnode, the interference pattern is not observed and is the circumcision of high spatial frequencies in the spectrum of the nonlinear amplification of noise.

Another advantage of lasers, built on the principle of pulse shaping with a calculated function of the mutual coherence of the radiation is a substantial simplification of the requirements for the optics.

In fact, for the "wiring" of the laser beam with divergence α through a complex optical system without impairment of its quality, it is necessary that the total wave aberration caused by the inhomogeneity of the optical medium according to the refractive index, the deviations of the surfaces of the optics used from ideal, thermal lenses, etc. that lead to the deterioration of the beam, to satisfy the condition:

where n is the refractive index of the medium, λ - wavelength radiation.

If the wavefront aberrations caused mainly by the deviation of the shape of the production of surfaces of optical elements from the ideal, which in optics it is customary to estimate the number of interference rings N on the base length D, the inequality (5) we get the following condition on the accuracy of surface treatment:

Here λ₀- wavelength radiation control, M - the number of optical surfaces in the optical is Eskom the path of the laser. For example, the diffraction beam (γ₁₂˜1) the value of N<1, for partially coherent radiation the value of N may be increased by (1/γ₁₂). Therefore, the requirements to the quality of processing of optical circuit elements of such a laser is significantly reduced, and with the exception of the schema, spatial filters, apodyterium diaphragms, and other devices used to maintain the quality of the beams, the manufacturing costs of the laser is several times smaller.

The drawing shows the optical scheme of high-power pulsed laser, which implements the method of forming a laser pulse with a calculated function of the mutual coherence of the radiation.

The scheme consists of the following components:

1. The master oscillator;

2. The system of formation of spatial-temporal characteristics of radiation;

3. The amplifier φ20;

4. The amplifier φ30;

5. The amplifier φ45;

6. The amplifier φ45.

Parameters of high-power pulsed laser is shown in the table.

The master oscillator generates a laser pulse with the given parameters on the form and duration, and spatial and temporal degree of coherence of the laser pulse.

The system of formation of spatial-temporal characteristics of laser radiation provides the desired contrast and the level of energy required to download amplification stages.

As the active elements of the amplifier stages are used rods with a length of 680 mm silicate glass brand voice-1. Amplifiers with the diameter of the active elements 20, 30, 45, 45 mm form a coherent optical circuit in which amplification of partially coherent radiation generated in the master oscillator and the formation of spatial-temporal characteristics of the laser pulse.

The proposed invention provides the following technical advantages:

1. The almost complete suppression of the speckle structure of the radiation pulse of nanosecond duration.

2. The developmental threshold of self-focusing is shifted toward larger values of the laser load active medium of the laser.

3. The absence of complex optical devices correction wavefront of the laser beam and a substantial simplification of the requirements for the optics.

4. The possibility of using longer active elements with large gains.

1. The method of formation of an intense laser pulse, including the creation and formation of spatial-temporal characteristics of the pulse, increased its suppression of self-focusing, characterized in that the shaping and amplification of the laser pulse produced by the calculation function of the mutual coherence

where 0<γ₁₂=λ/(α· (D)<1 - degree spatial coherence;

γ_τ=λ²/(δλ··τ) - the degree of temporal coherence;

τ - pulse duration;

λ - wavelength radiation;

α - the divergence of the laser beam;

D - aperture beam;

δλ - width line generation;

C is the speed of light.

2. Device for the formation of an intense laser pulse containing the master oscillator, the system of the formation of spatial-temporal characteristics of the laser pulse, amplifiers, characterized in that the amplifiers are placed sequentially in the increasing section reinforce the učka in accordance with the spatial-angular distribution of the radiation intensity, consistent with a calculated function of mutual coherence.