RU2106731C1 - Laser with changeable spectral and time characteristics - Google Patents

Abstract

FIELD: laser engineering. SUBSTANCE: laser has active medium and optical resonator consisting of spectral and time parts positioned on different sides of active medium. Time part of optical resonator includes blind flat mirror, in front of which comb-type mask is installed, blind spherical mirror and rotating blind flat mirror positioned in focus of given spherical mirror. Spectral part has flat output mirror, blind spherical mirror and diffraction grating mounted in focus of given spherical mirror. Two blind flat mirrors and optical screen arranged in front of these mirrors are located on both sides of flat output mirror. Flat blind mirror of resonator time part and flat output mirror of resonator spectral part are spaced from focal planes of respective spherical mirrors for definite distances which are determined from corresponding formulas. EFFECT: optimal effect of laser radiation on different materials. 1 dwg

Description

 The invention relates to laser technology and can be used in industry for the most optimal effect of laser radiation on various materials, initiation of chemical reactions, separation of isotopes, diagnostics of the atmosphere, etc.

The problem of creating highly efficient tunable multi-frequency lasers has long been a problem. The most optimal optical scheme (OS) for infrared (IR) lasers operating on vibrational-rotational transitions of CO, CO ₂ , HF and other molecules is known, which makes it easy to rearrange the generation spectrum of these lasers [1]. It consists of an active medium (AS), a partially reflecting (output) mirror, a diffraction grating (DR), a fully reflecting (blank) spherical mirror (SZ), and a blank flat mirror. However, the disadvantage of this laser is the lack of adjustment of its temporal characteristics, which is necessary to optimize the effect of laser radiation on various materials.

 One of the methods for controlling the temporal characteristics of laser radiation is the resonator Q-switching (MDR). A traditional OS for lasers with MDS based on the use of a rotating mirror is known. It consists of an AS, an output mirror, a fully reflecting rotating mirror (VZ) and a blind mirror [2]. To form a series of pulses in this circuit, a set of several blind mirrors (in terms of the number of pulses) is used.

 This OS is simple and reliable, but it has several disadvantages. First, in the case of using the multipulse mode, a large number of blind mirrors are required. Secondly, each mirror is configured (adjusted) independently, which is a laborious and lengthy process. Thirdly, in this OS, the on-time factor of the resonator Q factor increases or decreases only due to a change in the rotation speed of the mirror. In this case, the time during which the resonator is in the on state also changes in exactly the same way. Therefore, it is not possible to obtain a sharp leading edge of the generation pulse, irrespective of the duration of the pulse itself.

 The closest in technical essence to the invention is a laser with control of the spectral and temporal characteristics of radiation by intracavity spatial filtering [3]. It consists of a speaker system, a blind SZ, a DR, a fully reflecting SZ, a focusing SZ, and a deaf plane mirror. It allows you to simultaneously control the spectral and temporal characteristics of the radiation by using optical masks superimposed on a blank flat mirror. However, this laser has several disadvantages. First, the removal of radiation through the zeroth order of the DR is energetically disadvantageous, since no more than 10% of the radiation is output through it. Secondly, when the VZ is rotated even at a small angle from the axis of the resonator, the alignment of all spectral lines is violated, while the laser beams shift along the surface of a deaf plane mirror. Therefore, you cannot replace this mirror with an output one and display each line separately, as was done in [1]. Thirdly, the inconvenience when using the OS of this laser to create industrial lasers is also the fact that it is realized only when the time sweep of the cavity axis by a rotating mirror occurs in a plane parallel to the strokes of the DR. The main disadvantage of this scheme is the lack of the ability to independently control the temporal and spectral characteristics of laser radiation.

 In the scientific literature, a method has been proposed for increasing the power of a frequency-selective CO laser [4]. It is based on the fact that only one line of radiation is removed from the resonator, and the radiation of all the other lines is “blocked” in the resonator by means of fully reflecting mirrors. However, the practical implementation of this method has shown that it is required to block not all the remaining laser generation lines, but only strictly defined ones [5]. In addition, to increase the laser power, it is optimal to derive from the resonator not one but several adjacent lines.

 The objective of the invention is to increase the power of selective multi-frequency lasers and expand the capabilities of independent control of their spectral and temporal characteristics.

 According to the invention, the independence of the control of spectral and temporal characteristics is achieved by using an optical resonator (OR), divided into spectral and temporal parts, which are spaced on different sides of the speaker. In this case, the output of laser radiation can be realized both from the spectral part of the optical path and from the temporary side by replacing the corresponding deaf mirror with an output one.

According to the invention, increasing the power of selective multi-frequency pulsed lasers is carried out:
firstly, by changing the configuration of the resonator (flat, confocal, convex, etc.) by adjusting the effective focal lengths of the spectral and temporal parts of the OR using the same optical elements;
secondly, by eliminating the rigid attachment of the plane of rotation of the mirror to the orientation of the DR;
thirdly, due to the formation of a series of laser radiation pulses without using a set of blind mirrors;
fourthly, through the use of various physical mechanisms that influence the formation of the inverse population in the AS.

,
где
L₁ - расстояние от глухого зеркала до фокальной плоскости СЗ, входящего в состав временной части ОР;
L₂ - расстояние от выходного зеркала до фокальной плоскости СЗ, входящего в состав спектральной части ОР;
R₁ - радиус кривизны СЗ, входящего в состав временной части ОР;
R₂ - радиус кривизны СЗ, входящего в состав спектральной части ОР;
F $\genfrac{}{}{0ex}{}{\text{эф}}{\text{1}}$ - эффективное фокусное расстояние временной части ОР;
F $\genfrac{}{}{0ex}{}{\text{эф}}{\text{2}}$ - эффективное фокусное расстояние спектральной части ОР.The first point of solving this problem is carried out by establishing a flat output and blind mirrors at a certain distance from the SZ included in each of the parts of the resonator. For example, if a blind mirror is installed in the temporary part of the OP, then it is located at a certain distance L ₁ from the focal plane of the SZ with a radius of curvature R ₁ that is part of this part of the OP. Accordingly, the output plane mirror is installed at a certain distance L ₂ from the focal plane of the SZ with a radius of curvature R ₂ , which is part of the spectral part of the OP. In this time-OT parts PR and DR spectral portion installed strictly in the focal planes SOC data t. E. At distances R _1/2 and R _2/2 from them, respectively. Distances L ₁ and L _{2 are} determined by the formula:

,
Where
L ₁ is the distance from the deaf mirror to the focal plane of the NW, which is part of the temporary part of the OR;
L ₂ is the distance from the output mirror to the focal plane of the NW, which is part of the spectral part of the OR;
R ₁ is the radius of curvature of the SZ, which is part of the temporary part of the PR;
R ₂ is the radius of curvature of the SZ, which is part of the spectral part of the OP;
F $\genfrac{}{}{0ex}{}{\text{ef}}{\text{1}}$ - effective focal length of the time part of the OP;
F $\genfrac{}{}{0ex}{}{\text{ef}}{\text{2}}$ - effective focal length of the spectral part of the OR.

Thus, only by moving the flat mirrors can any configuration of the resonator be obtained (flat, confocal, convex-concave, etc.), which is determined by the ratio between the effective focal lengths F $\genfrac{}{}{0ex}{}{\text{ef}}{\text{1}}$ , F $\genfrac{}{}{0ex}{}{\text{ef}}{\text{2}}$ and the effective cavity length equal to the optical path between spherical mirrors.

 The location of the DR and the VZ in the focal planes of the corresponding SZ allows us to solve the second point of the problem: eliminating the rigid attachment of the plane of rotation of the mirror to the orientation of the DR. In the proposed PR, the laser scanning plane in the time part does not require rigid binding to the orientation of the DR and can be selected from the conditions of convenience of a particular implementation of the OR.

In the proposed OR, in comparison with the traditional MDS scheme, the maximum time during which the cavity is in the aligned state can be increased several times (at the same speed of rotation of the explosive and for a similar geometry of the laser radiation field inside the AS). The ratio of these times is calculated by the formula:
t / t _o = θ / θ _p ,,
Where
t is the time during which the resonator is in the aligned state in the proposed scheme,
t ₀ is the corresponding time in the traditional scheme,
θ is the characteristic value of the angular divergence of the laser radiation, determined by the length, configuration and aperture of the laser resonator (approximately 10 ^-4 <θ <10 ^-2 rad),
θ _p is the angle determining the fulfillment of the condition of the paraxial approximation (absence of optical aberrations) during a temporary scan of the laser beam along the SZ θ _p ≈ 10 ^-1 rad).

 Thus, this ratio can actually reach several tens, which allows us to solve the third point of the task: the formation of a series of short pulses. This is achieved by imposing an optical mask in the form of a comb on a flat flat mirror, the shape of which is determined by the required pulse duration and duty cycle.

 According to the invention, the fourth point of the task in the proposed PR is solved as follows. In its spectral part, a simple output mirror is replaced by a composite one. An output mirror of the resonator is installed in the center of the laser beam coming from the SZ, and the rest of the laser radiation is blocked by two blind mirrors. The width of the gap between the locking mirrors is determined by the width of the required generation spectrum. As a physical experiment showed, it is possible to increase the power of laser radiation not just by “locking” the rest of the laser radiation, but by blocking the generation of individual lines and entire spectral ranges inside the resonator. The choice of locked generation lines left in the resonator depends on the mechanism of formation of the inverse population in the speakers of this laser and determines the shape of the optical screen mounted in front of the locking mirrors, which can completely close them when there is only laser radiation specified in frequency in the resonator and output from it .

 The drawing shows a laser with tunable spectral and temporal characteristics.

 This laser contains an active medium 1 and an optical resonator (OR), divided into spectral and temporal parts.

The temporary part of the OR consists of a rotating flat fully reflecting mirror 2: a focusing spherical mirror 3, remote at a distance equal to half its radius of curvature from mirror 2; a fully reflective flat mirror 4 mounted at a distance L ₁ behind the focal plane of the mirror 3, and an optical mask 5 mounted in front of the mirror 4.

The spectral part of the OP consists of a diffraction grating 6; a focusing spherical mirror 7 mounted at a distance equal to half its radius of curvature from the diffraction grating 6; partially reflecting flat mirror 8; fully reflecting flat "locking" mirrors 9, 10 and an optical screen 11 mounted in front of the mirrors 9 and 10. The mirrors 8, 9 and 10 are installed at a certain distance L ₂ from the focal plane of the mirror 7, while the width of the slit 8 between the locking mirrors 9 and 10 is determined by the required width of the laser generation spectrum and the characteristics of the selected DR.

For a specific embodiment of the invention, a pulsed cryogenic electroionization CO laser was used with a gas mixture temperature of 110 K and a density of 0.25 Amag. For spherical radiation, copper spherical mirrors with gold sputtering and a radius of curvature of 2 m were used. Deaf flat mirrors are made of glass with copper sputtering, and a flat partially reflecting mirror is made of CaF ₂ with dielectric sputtering, providing a reflection coefficient of 30% (in the wavelength range 4.5-6.1 μm). For the MDR, the optical head of a high-speed photorecorder with a continuously variable rotation speed of a flat mirror with aluminum coating from 0 to 60,000 rpm was used.

 The principle of operation of the proposed laser is based on the fact that the optical resonator described above generates highly coherent laser radiation from spontaneous AC radiation with certain spectral and temporal characteristics that can be controlled independently.

 The temporal characteristics of laser radiation (the duration of the generation pulse, the shape, quantity and time distance between pulses for the mode of formation of a series of pulses) are regulated by changing the rotation speed of mirror 2, changing the distance from mirror 4 to mirror 3, and using various optical masks 5.

 The spectral characteristics of the output laser radiation (range and width of the generation spectrum) are regulated by changing the angle of rotation of the DR and the width of the slit S between the mirrors 9 and 10.

 The energy characteristics of a multi-frequency laser operating in selective mode with MDS can be changed by adjusting the spectral composition of the laser radiation locked in the cavity by installing an optical screen 11 in front of the locking mirrors 9 and 10.

Thus, the invention allows:
independently control both temporal and spectral characteristics of the output laser radiation;
to realize the possibility of changing the configuration of the resonator (flat, confocal, convex-concave, etc.) not by replacing the optical elements, but by adjusting the effective focal lengths of the spectral and temporal parts of the OR using the same optical elements:
eliminate the rigid binding of the plane of rotation of the mirror to the orientation of the DR;
to form a series of pulses of laser radiation without using a set of blind mirrors;
increase the power and expand the ability to control the spectral characteristics of multi-frequency lasers through the use of various physical mechanisms that affect the formation of the inverse population in the AS.

 Sources of information.

 1. Soloukhin R.I., Jacobi Yu.A., Vyazovich E.I. USSR author's certificate N 594842 dated 04/05/76, BI N 11, 1979.

 2. Ananyev V.Yu., Basov N.G., Ionin A.A., Kuchaev A.V., Lytkin A.P., Petrunin V.V., Sinitsyn D.V. Increase in the efficiency of CO laser radiation Q-modulated by generating a series of pulses. - Quantum Electronics, 12, N 8.1666-1670 (1985).

 Z. Gutin M.A. Laser with control of the spectral and temporal characteristics of radiation by intracavity spatial filtering. Optics and Spectroscopy, 64, No. 6, 1190 (1988).

 4. Gutin M.A., Kolchenko A.P. A method of frequency-selective energy removal from a resonator for line selection in a Q-switched Q-switched laser. Quantum Electronics, 13, N 1, 1986.

 5. Ananyev V. Yu., Danilychev V.A., Ionin A.A., Lytkin A.P. The formation of the vibrational-rotational structure of the spectrum of the generation of EI CO-lasers. - Quantum Electronics, 14, N 10, 1974-1980 (1987).

Claims (1)

A laser with tunable spectral and temporal characteristics, containing an active medium and an optical resonator, consisting of a temporal part, including a deaf plane mirror, a deaf spherical mirror with a radius of curvature R _1, and a rotating deaf plane mirror mounted at the focus of a given spherical mirror, and a spectral portion containing the flat output mirror blind spherical mirror with a radius of curvature R ₂ and a diffraction grating disposed at the focus of a spherical mirror, wherein h The time and spectral parts of the optical resonator are located on opposite sides of the active medium, a comb mask is installed in front of the flat blind mirror of the temporary part of the optical resonator, and two additional blind flat mirrors mounted on both sides of the output mirror and an optical screen are introduced into its spectral part located in front of these mirrors, while the flat blind mirror of the temporal part of the resonator and the flat output mirror of the spectral part of the resonator are separated from the focal planes minutes corresponding spherical mirrors at distance L ₁ and L _2, are defined by the formula

where L ₁ is the distance from the deaf mirror to the focal plane of the spherical mirror, which is part of the time part of the optical resonator;
L ₂ is the distance from the output mirror to the focal plane of a spherical mirror that is part of the spectral part of the optical resonator;
R ₁ is the radius of curvature of a spherical mirror, which is part of the temporal part of the optical resonator;
R ₂ is the radius of curvature of a spherical mirror, which is part of the spectral part of the optical resonator;
F $\genfrac{}{}{0ex}{}{\text{ef}}{\text{1}}$ - effective focal length of the time part of the optical resonator;
F $\genfrac{}{}{0ex}{}{\text{ef}}{\text{2}}$ - effective focal length of the spectral part of the optical resonator.